diff --git a/vyper/semantics/analysis/module.py b/vyper/semantics/analysis/module.py
index e50c3e6d6f..9304eb3ded 100644
--- a/vyper/semantics/analysis/module.py
+++ b/vyper/semantics/analysis/module.py
@@ -342,7 +342,14 @@ def visit_ImplementsDecl(self, node):
         type_ = type_from_annotation(node.annotation)
 
         if not isinstance(type_, InterfaceT):
-            raise StructureException("not an interface!", node.annotation)
+            msg = "Not an interface!"
+            hint = None
+            if isinstance(type_, ModuleT):
+                path = type_._module.path
+                msg += " (Since vyper v0.4.0, interface files are required"
+                msg += " to have a .vyi suffix.)"
+                hint = f"try renaming `{path}` to `{path}i`"
+            raise StructureException(msg, node.annotation, hint=hint)
 
         type_.validate_implements(node)
 
@@ -627,6 +634,9 @@ def _load_import(self, node: vy_ast.VyperNode, level: int, module_str: str, alia
     def _load_import_helper(
         self, node: vy_ast.VyperNode, level: int, module_str: str, alias: str
     ) -> Any:
+        if module_str.startswith("vyper.interfaces"):
+            hint = "try renaming `vyper.interfaces` to `ethereum.ercs`"
+            raise ModuleNotFound(module_str, hint=hint)
         if _is_builtin(module_str):
             return _load_builtin_import(level, module_str)
 
@@ -724,7 +734,7 @@ def _is_builtin(module_str):
 
 def _load_builtin_import(level: int, module_str: str) -> InterfaceT:
     if not _is_builtin(module_str):
-        raise ModuleNotFoundError(f"Not a builtin: {module_str}") from None
+        raise ModuleNotFoundError(f"Not a builtin: {module_str}")
 
     builtins_path = vyper.builtins.interfaces.__path__[0]
     # hygiene: convert to relpath to avoid leaking user directory info
diff --git a/vyper/semantics/types/utils.py b/vyper/semantics/types/utils.py
index c6a4531df8..96c661021f 100644
--- a/vyper/semantics/types/utils.py
+++ b/vyper/semantics/types/utils.py
@@ -3,6 +3,7 @@
 from vyper import ast as vy_ast
 from vyper.exceptions import (
     ArrayIndexException,
+    CompilerPanic,
     InstantiationException,
     InvalidType,
     StructureException,
@@ -158,6 +159,12 @@ def _type_from_annotation(node: vy_ast.VyperNode) -> VyperType:
         # call from_annotation to produce a better error message.
         typ_.from_annotation(node)
 
+    if hasattr(typ_, "module_t"):  # it's a ModuleInfo
+        typ_ = typ_.module_t
+
+    if not isinstance(typ_, VyperType):
+        raise CompilerPanic("Not a type: {typ_}", node)
+
     return typ_