diff --git a/lib/error.js b/lib/error.js
index dcbb7e6e..4b1f53dc 100644
--- a/lib/error.js
+++ b/lib/error.js
@@ -15,11 +15,12 @@ function appendIfExist(base, target) {
 }
 
 function contextAsText(node) {
-  const hierarchy = [node];
-  while (node && node.parent) {
-    const { parent } = node;
-    hierarchy.unshift(parent);
-    node = parent;
+  const hierarchy = [];
+  while (node) {
+    if (node.type) {
+      hierarchy.unshift(node);
+    }
+    node = node.parent;
   }
   return hierarchy.map((n) => appendIfExist(n.type, n.name)).join(" -> ");
 }
@@ -83,12 +84,11 @@ function error(
 
   const contextType = kind === "Syntax" ? "since" : "inside";
   const inSourceName = source.name ? ` in ${source.name}` : "";
-  const grammaticalContext =
-    current && current.name
-      ? `, ${contextType} \`${current.partial ? "partial " : ""}${contextAsText(
-          current
-        )}\``
-      : "";
+  const grammaticalContext = current
+    ? `, ${contextType} \`${current.partial ? "partial " : ""}${contextAsText(
+        current
+      )}\``
+    : "";
   const context = `${kind} error at line ${line}${inSourceName}${grammaticalContext}:\n${sourceContext}`;
   return {
     message: `${context} ${message}`,
diff --git a/lib/productions/argument.js b/lib/productions/argument.js
index 1882a05d..c86868ca 100644
--- a/lib/productions/argument.js
+++ b/lib/productions/argument.js
@@ -69,15 +69,7 @@ export class Argument extends Base {
       useNullableInner: true,
     });
     if (result) {
-      if (this.idlType.nullable) {
-        const message = `Dictionary arguments cannot be nullable.`;
-        yield validationError(
-          this.tokens.name,
-          this,
-          "no-nullable-dict-arg",
-          message
-        );
-      } else if (!this.optional) {
+      if (!this.optional) {
         if (
           this.parent &&
           !dictionaryIncludesRequiredField(result.dictionary, defs) &&
diff --git a/lib/productions/type.js b/lib/productions/type.js
index 1746499b..0528c3ec 100644
--- a/lib/productions/type.js
+++ b/lib/productions/type.js
@@ -203,22 +203,21 @@ for more information.`;
       });
     }
 
+    const ref = !this.union && defs.unique.get(this.idlType);
+    const target = ref && ref.type === "typedef" ? ref.idlType : this;
+
     /*
      * If a union is nullable, its subunions cannot include a dictionary
      * If not, subunions may include dictionaries if each union is not nullable
      */
-    const typedef = !this.union && defs.unique.get(this.idlType);
-    const target = this.union
-      ? this
-      : typedef && typedef.type === "typedef"
-      ? typedef.idlType
-      : undefined;
     if (target && this.nullable) {
       // do not allow any dictionary
-      const { reference } = idlTypeIncludesDictionary(target, defs) || {};
+      const { reference } =
+        idlTypeIncludesDictionary(target, defs, { useNullableInner: true }) ||
+        {};
       if (reference) {
         const targetToken = (this.union ? reference : this).tokens.base;
-        const message = "Nullable union cannot include a dictionary type.";
+        const message = "A dictionary type can never be nullable";
         yield validationError(
           targetToken,
           this,
@@ -226,11 +225,10 @@ for more information.`;
           message
         );
       }
-    } else {
-      // allow some dictionary
-      for (const subtype of this.subtype) {
-        yield* subtype.validate(defs);
-      }
+    }
+
+    for (const subtype of this.subtype) {
+      yield* subtype.validate(defs);
     }
   }
 
diff --git a/test/invalid/baseline/argument-dict-nullable.txt b/test/invalid/baseline/argument-dict-nullable.txt
index 9ade1218..74464e36 100644
--- a/test/invalid/baseline/argument-dict-nullable.txt
+++ b/test/invalid/baseline/argument-dict-nullable.txt
@@ -1,24 +1,27 @@
-(no-nullable-dict-arg) Validation error at line 13 in argument-dict-nullable.webidl, inside `interface X -> operation x1 -> argument dict`:
+(no-nullable-union-dict) Validation error at line 13 in argument-dict-nullable.webidl, inside `interface X -> operation x1 -> argument dict -> argument-type`:
+  undefined x1(optional Dict? dict);
+                        ^ A dictionary type can never be nullable
+(dict-arg-default) Validation error at line 13 in argument-dict-nullable.webidl, inside `interface X -> operation x1 -> argument dict`:
  x1(optional Dict? dict);
-                   ^ Dictionary arguments cannot be nullable.
-(no-nullable-dict-arg) Validation error at line 14 in argument-dict-nullable.webidl, inside `interface X -> operation x2 -> argument dict`:
- x2(optional Dict? dict = {})
-                   ^ Dictionary arguments cannot be nullable.
-(no-nullable-union-dict) Validation error at line 15 in argument-dict-nullable.webidl:
+                   ^ Optional dictionary arguments must have a default value of `{}`.
+(no-nullable-union-dict) Validation error at line 14 in argument-dict-nullable.webidl, inside `interface X -> operation x2 -> argument dict -> argument-type`:
+  undefined x2(optional Dict? dict = {
+                        ^ A dictionary type can never be nullable
+(no-nullable-union-dict) Validation error at line 15 in argument-dict-nullable.webidl, inside `interface X -> operation y2 -> argument union -> argument-type`:
 (optional (boolean or Dict)? union =
-                      ^ Nullable union cannot include a dictionary type.
-(no-nullable-dict-arg) Validation error at line 15 in argument-dict-nullable.webidl, inside `interface X -> operation y2 -> argument union`:
-boolean or Dict)? union = {})
-                  ^ Dictionary arguments cannot be nullable.
-(no-nullable-union-dict) Validation error at line 16 in argument-dict-nullable.webidl:
+                      ^ A dictionary type can never be nullable
+(no-nullable-union-dict) Validation error at line 16 in argument-dict-nullable.webidl, inside `interface X -> operation y3 -> argument union -> argument-type`:
+(optional (boolean or Dict?) union)
+                      ^ A dictionary type can never be nullable
+(no-nullable-union-dict) Validation error at line 17 in argument-dict-nullable.webidl, inside `interface X -> operation z2 -> argument union -> argument-type`:
   undefined z2(optional Union? union = {
-                        ^ Nullable union cannot include a dictionary type.
-(no-nullable-dict-arg) Validation error at line 16 in argument-dict-nullable.webidl, inside `interface X -> operation z2 -> argument union`:
- z2(optional Union? union = {})
-                    ^ Dictionary arguments cannot be nullable.
-(no-nullable-dict-arg) Validation error at line 17 in argument-dict-nullable.webidl, inside `interface X -> operation r -> argument req`:
+                        ^ A dictionary type can never be nullable
+(no-nullable-union-dict) Validation error at line 18 in argument-dict-nullable.webidl, inside `interface X -> operation r -> argument req -> argument-type`:
   undefined r(Required? req);
-                        ^ Dictionary arguments cannot be nullable.
-(no-nullable-dict-arg) Validation error at line 19 in argument-dict-nullable.webidl, inside `interface X -> iterable -> argument dict`:
+              ^ A dictionary type can never be nullable
+(no-nullable-union-dict) Validation error at line 20 in argument-dict-nullable.webidl, inside `interface X -> iterable -> argument dict -> argument-type`:
+<DOMString>(optional Dict? dict);
+                     ^ A dictionary type can never be nullable
+(dict-arg-default) Validation error at line 20 in argument-dict-nullable.webidl, inside `interface X -> iterable -> argument dict`:
 >(optional Dict? dict);
-                 ^ Dictionary arguments cannot be nullable.
+                 ^ Optional dictionary arguments must have a default value of `{}`.
diff --git a/test/invalid/baseline/argument-dict-optional.txt b/test/invalid/baseline/argument-dict-optional.txt
index 35b5b244..32cf41cc 100644
--- a/test/invalid/baseline/argument-dict-optional.txt
+++ b/test/invalid/baseline/argument-dict-optional.txt
@@ -1,3 +1,6 @@
+(no-nullable-union-dict) Validation error at line 22 in argument-dict-optional.webidl, inside `typedef NullableUnion -> typedef-type`:
+typedef (DOMString or Optional?) NullableUnion;
+                      ^ A dictionary type can never be nullable
 (dict-arg-optional) Validation error at line 25 in argument-dict-optional.webidl, inside `interface mixin Container -> operation op1 -> argument shouldBeOptional`:
   undefined op1(Optional shouldBeOptional);
                          ^ Dictionary argument must be optional if it has no required fields
diff --git a/test/invalid/baseline/nullable-union-dictionary.txt b/test/invalid/baseline/nullable-union-dictionary.txt
index e8efd350..e17f9871 100644
--- a/test/invalid/baseline/nullable-union-dictionary.txt
+++ b/test/invalid/baseline/nullable-union-dictionary.txt
@@ -1,39 +1,42 @@
-(no-nullable-union-dict) Validation error at line 4 in nullable-union-dictionary.webidl:
+(no-nullable-union-dict) Validation error at line 4 in nullable-union-dictionary.webidl, inside `typedef NullableBooleanDict -> typedef-type`:
 typedef (boolean or Dict)? NullableBooleanDict;
-                    ^ Nullable union cannot include a dictionary type.
-(no-nullable-union-dict) Validation error at line 6 in nullable-union-dictionary.webidl:
+                    ^ A dictionary type can never be nullable
+(no-nullable-union-dict) Validation error at line 6 in nullable-union-dictionary.webidl, inside `typedef NullableNestedBooleanDict -> typedef-type`:
 boolean or (short or Dict))? NullableNestedBooleanDict
-                     ^ Nullable union cannot include a dictionary type.
-(no-nullable-union-dict) Validation error at line 7 in nullable-union-dictionary.webidl:
+                     ^ A dictionary type can never be nullable
+(no-nullable-union-dict) Validation error at line 7 in nullable-union-dictionary.webidl, inside `typedef NestedNullableBooleanDict -> typedef-type`:
 boolean or (short or Dict)?) NestedNullableBooleanDict
-                     ^ Nullable union cannot include a dictionary type.
-(no-nullable-union-dict) Validation error at line 8 in nullable-union-dictionary.webidl:
+                     ^ A dictionary type can never be nullable
+(no-nullable-union-dict) Validation error at line 8 in nullable-union-dictionary.webidl, inside `typedef ReferencingNullableBooleanDict -> typedef-type`:
 typedef BooleanDict? ReferencingNullableBooleanDict;
-        ^ Nullable union cannot include a dictionary type.
-(no-nullable-union-dict) Validation error at line 9 in nullable-union-dictionary.webidl:
+        ^ A dictionary type can never be nullable
+(no-nullable-union-dict) Validation error at line 9 in nullable-union-dictionary.webidl, inside `typedef RecursiveBooleanDict -> typedef-type`:
 typedef (boolean or RecursiveBooleanDict? or Dict)
-                    ^ Nullable union cannot include a dictionary type.
-(no-nullable-union-dict) Validation error at line 16 in nullable-union-dictionary.webidl:
+                    ^ A dictionary type can never be nullable
+(no-nullable-union-dict) Validation error at line 14 in nullable-union-dictionary.webidl, inside `typedef NullableBooleanNullableDict -> typedef-type`:
+typedef (boolean or Dict?)? NullableBooleanNullableDict
+                    ^ A dictionary type can never be nullable
+(no-nullable-union-dict) Validation error at line 16 in nullable-union-dictionary.webidl, inside `callback Callback -> return-type`:
  Callback = (boolean or Dict)? ()
-                        ^ Nullable union cannot include a dictionary type.
-(no-nullable-union-dict) Validation error at line 19 in nullable-union-dictionary.webidl:
+                        ^ A dictionary type can never be nullable
+(no-nullable-union-dict) Validation error at line 19 in nullable-union-dictionary.webidl, inside `interface Interface -> operation op -> return-type`:
   (boolean or Dict)? op(
-              ^ Nullable union cannot include a dictionary type.
-(no-nullable-union-dict) Validation error at line 20 in nullable-union-dictionary.webidl:
+              ^ A dictionary type can never be nullable
+(no-nullable-union-dict) Validation error at line 20 in nullable-union-dictionary.webidl, inside `interface Interface -> operation voidOp -> argument arg -> argument-type`:
  voidOp((boolean or Dict)? arg)
-                    ^ Nullable union cannot include a dictionary type.
-(no-nullable-dict-arg) Validation error at line 20 in nullable-union-dictionary.webidl, inside `interface Interface -> operation voidOp -> argument arg`:
+                    ^ A dictionary type can never be nullable
+(dict-arg-optional) Validation error at line 20 in nullable-union-dictionary.webidl, inside `interface Interface -> operation voidOp -> argument arg`:
 boolean or Dict)? arg);
-                  ^ Dictionary arguments cannot be nullable.
-(no-nullable-union-dict) Validation error at line 21 in nullable-union-dictionary.webidl:
+                  ^ Dictionary argument must be optional if it has no required fields
+(no-nullable-union-dict) Validation error at line 21 in nullable-union-dictionary.webidl, inside `interface Interface -> attribute attr -> attribute-type`:
   attribute (boolean or Dict)? attr;
-                        ^ Nullable union cannot include a dictionary type.
+                        ^ A dictionary type can never be nullable
 (attr-invalid-type) Validation error at line 21 in nullable-union-dictionary.webidl, inside `interface Interface -> attribute attr`:
   attribute (boolean or Dict
   ^ Attributes cannot accept dictionary types.
-(no-nullable-union-dict) Validation error at line 23 in nullable-union-dictionary.webidl:
+(no-nullable-union-dict) Validation error at line 23 in nullable-union-dictionary.webidl, inside `interface Interface -> iterable`:
   iterable<(boolean or Dict)?>;
-                       ^ Nullable union cannot include a dictionary type.
-(no-nullable-union-dict) Validation error at line 27 in nullable-union-dictionary.webidl:
+                       ^ A dictionary type can never be nullable
+(no-nullable-union-dict) Validation error at line 27 in nullable-union-dictionary.webidl, inside `dictionary AnotherDict -> field dict -> dictionary-type`:
   (boolean or Dict)? dict;
-              ^ Nullable union cannot include a dictionary type.
+              ^ A dictionary type can never be nullable
diff --git a/test/invalid/baseline/operation-nameless.txt b/test/invalid/baseline/operation-nameless.txt
index 97ee26a4..c7dbe69b 100644
--- a/test/invalid/baseline/operation-nameless.txt
+++ b/test/invalid/baseline/operation-nameless.txt
@@ -1,6 +1,6 @@
-(incomplete-op) Validation error at line 3 in operation-nameless.webidl:
+(incomplete-op) Validation error at line 3 in operation-nameless.webidl, inside `interface Anonymity -> operation`:
   copy();
       ^ Regular or static operations must have both a return type and an identifier.
-(incomplete-op) Validation error at line 4 in operation-nameless.webidl:
+(incomplete-op) Validation error at line 4 in operation-nameless.webidl, inside `interface Anonymity -> operation`:
   static create();
                ^ Regular or static operations must have both a return type and an identifier.
diff --git a/test/invalid/baseline/void-keyword.txt b/test/invalid/baseline/void-keyword.txt
index 37f4c118..692fb3a7 100644
--- a/test/invalid/baseline/void-keyword.txt
+++ b/test/invalid/baseline/void-keyword.txt
@@ -1,3 +1,3 @@
-(replace-void) Validation error at line 3 in void-keyword.webidl:
+(replace-void) Validation error at line 3 in void-keyword.webidl, inside `interface Foo -> operation foo -> return-type`:
   void foo();
   ^ `void` is now replaced by `undefined`. Refer to the [relevant GitHub issue](https://github.com/heycam/webidl/issues/60) for more information.
diff --git a/test/invalid/idl/argument-dict-nullable.webidl b/test/invalid/idl/argument-dict-nullable.webidl
index 087fe8ea..fd7bac13 100644
--- a/test/invalid/idl/argument-dict-nullable.webidl
+++ b/test/invalid/idl/argument-dict-nullable.webidl
@@ -13,6 +13,7 @@ interface X {
   undefined x1(optional Dict? dict);
   undefined x2(optional Dict? dict = {});
   undefined y2(optional (boolean or Dict)? union = {});
+  undefined y3(optional (boolean or Dict?) union);
   undefined z2(optional Union? union = {});
   undefined r(Required? req);