diff --git a/src/lib/types/definition.js b/src/lib/types/definition.js
index 9a27599..ade70ce 100644
--- a/src/lib/types/definition.js
+++ b/src/lib/types/definition.js
@@ -281,8 +281,17 @@ export class SchemaDefinition {
                     // Merge all properties from s into t, joining arrays and objects
                     for (let skey of Object.keys(s)) {
                         const tkey = skey.toLowerCase();
-                        if (Array.isArray(t[tkey]) && Array.isArray(s[skey])) t[tkey].push(...s[skey]);
+                        
+                        // If source is an array...
+                        if (Array.isArray(s[skey])) {
+                            // ...and target is an array, merge them...
+                            if (Array.isArray(t[tkey])) t[tkey].push(...s[skey]);
+                            // ...otherwise, make target an array
+                            else t[tkey] = [...s[skey]];
+                        }
+                        // If source is a primitive value, copy it
                         else if (s[skey] !== Object(s[skey])) t[tkey] = s[skey];
+                        // Finally, if source is neither an array nor primitive, merge it
                         else t[tkey] = merge(t[tkey] ?? {}, s[skey]);
                     }
                     
@@ -297,7 +306,7 @@ export class SchemaDefinition {
                         // Coerce the mixed value, using only namespaced attributes for this extension
                         target[name] = attribute.coerce(mixedSource, direction, basepath, [Object.keys(filter ?? {})
                             .filter(k => k.startsWith(`${name}:`))
-                            .reduce((res, key) => (((res[key.replace(`${name}:`, "")] = filter[key]) || true) && res), {})
+                            .reduce((res, key) => Object.assign(res, {[key.replace(`${name}:`, "")]: filter[key]}), {})
                         ]);
                     } catch (ex) {
                         // Rethrow exception with added context
diff --git a/test/hooks/schemas.js b/test/hooks/schemas.js
index d44c04d..3d93fed 100644
--- a/test/hooks/schemas.js
+++ b/test/hooks/schemas.js
@@ -107,10 +107,34 @@ export default {
             }
         });
         
+        // https://github.com/scimmyjs/scimmy/issues/12
+        it("should coerce complex multi-value attributes in schema extensions", async () => {
+            const {constructor = {}} = await fixtures;
+            const subAttribute = new Attribute("string", "name");
+            const attribute = new Attribute("complex", "agencies", {multiValued: true}, [subAttribute]);
+            const extension = new SchemaDefinition("Extension", "urn:ietf:params:scim:schemas:Extension", "", [attribute]);
+            const source = {...constructor, [extension.id]: {[attribute.name]: [{[subAttribute.name]: "value"}]}};
+            
+            try {
+                // Add the extension to the target
+                TargetSchema.extend(extension);
+                
+                // Construct an instance to test against, and get actual value for comparison
+                const instance = new TargetSchema(source);
+                const actual = JSON.parse(JSON.stringify(instance[extension.id][attribute.name]));
+                
+                assert.deepStrictEqual(actual, source[extension.id][attribute.name],
+                    "Schema instance did not coerce complex multi-value attributes from schema extension");
+            } finally {
+                // Remove the extension so it doesn't interfere later
+                TargetSchema.truncate("urn:ietf:params:scim:schemas:Extension");
+            }
+        });
+        
         it("should expect errors in extension schema coercion to be rethrown as SCIMErrors", async () => {
+            const {constructor = {}} = await fixtures;
             const attributes = [new Attribute("string", "testValue")];
             const extension = new SchemaDefinition("Extension", "urn:ietf:params:scim:schemas:Extension", "", attributes);
-            const {constructor = {}} = await fixtures;
             const source = {...constructor, [`${extension.id}:testValue`]: "a string"};
             
             try {
@@ -145,8 +169,8 @@ export default {
             ];
             
             // Get the extension and the source data ready
-            const extension = new SchemaDefinition("Extension", "urn:ietf:params:scim:schemas:Extension", "", attributes);
             const {constructor = {}} = await fixtures;
+            const extension = new SchemaDefinition("Extension", "urn:ietf:params:scim:schemas:Extension", "", attributes);
             const source = {
                 ...constructor,
                 [`${extension.id}:testValue.stringValue`]: "a string",