diff --git a/include/swift/AST/DiagnosticsParse.def b/include/swift/AST/DiagnosticsParse.def
index 9a99e0dd2bb..53e71b410ae 100644
--- a/include/swift/AST/DiagnosticsParse.def
+++ b/include/swift/AST/DiagnosticsParse.def
@@ -2156,6 +2156,10 @@ ERROR(init_accessor_is_not_on_property,none,
       "init accessors could only be associated with properties",
       ())
 
+ERROR(init_accessor_is_not_in_the_primary_declaration,none,
+      "init accessors could only be declared in the primary declaration",
+      ())
+
 ERROR(missing_storage_restrictions_attr_label,none,
       "missing label in @storageRestrictions attribute", ())
 
diff --git a/lib/Parse/ParseDecl.cpp b/lib/Parse/ParseDecl.cpp
index 060ea5441e5..706611022ac 100644
--- a/lib/Parse/ParseDecl.cpp
+++ b/lib/Parse/ParseDecl.cpp
@@ -8432,8 +8432,12 @@ void Parser::ParsedAccessors::classify(Parser &P, AbstractStorageDecl *storage,
   }
 
   if (Init) {
-    if (!storage->getDeclContext()->getSelfNominalTypeDecl() ||
-        isa<SubscriptDecl>(storage)) {
+    if (storage->getDeclContext()->getContextKind() ==
+        DeclContextKind::ExtensionDecl) {
+      P.diagnose(Init->getLoc(),
+                 diag::init_accessor_is_not_in_the_primary_declaration);
+    } else if (!storage->getDeclContext()->getSelfNominalTypeDecl() ||
+               isa<SubscriptDecl>(storage)) {
       P.diagnose(Init->getLoc(), diag::init_accessor_is_not_on_property);
     }
   }