diff --git a/pkg/_fe_analyzer_shared/test/inheritance/data/top_merge_typedef.dart b/pkg/_fe_analyzer_shared/test/inheritance/data/top_merge_typedef.dart
new file mode 100644
index 0000000..a3c4ea0
--- /dev/null
+++ b/pkg/_fe_analyzer_shared/test/inheritance/data/top_merge_typedef.dart
@@ -0,0 +1,46 @@
+// Copyright (c) 2021, the Dart project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/*library: nnbd=true*/
+
+typedef Typedef1 = void Function();
+typedef Typedef2 = dynamic Function();
+
+/*class: A:A,Object*/
+abstract class A {
+  /*member: A.method1:void Function(void Function())*/
+  void method1(void Function() f);
+
+  /*member: A.method2:void Function(void Function())*/
+  void method2(Typedef1 f);
+
+  /*member: A.method3:void Function(void Function())*/
+  void method3(Typedef1 f);
+
+  /*member: A.method4:void Function(void Function())*/
+  void method4(void Function() f);
+}
+
+/*class: B:B,Object*/
+abstract class B {
+  /*member: B.method1:void Function(dynamic Function())*/
+  void method1(dynamic Function() f);
+
+  /*member: B.method2:void Function(dynamic Function())*/
+  void method2(Typedef2 f);
+
+  /*member: B.method3:void Function(dynamic Function())*/
+  void method3(dynamic Function() f);
+
+  /*member: B.method4:void Function(dynamic Function())*/
+  void method4(Typedef2 f);
+}
+
+/*class: C:A,B,C,Object*/
+abstract class C implements A, B {
+  /*member: C.method1:void Function(Object? Function())*/
+  /*member: C.method2:void Function(Object? Function())*/
+  /*member: C.method3:void Function(Object? Function())*/
+  /*member: C.method4:void Function(Object? Function())*/
+}
diff --git a/pkg/front_end/lib/src/fasta/kernel/combined_member_signature.dart b/pkg/front_end/lib/src/fasta/kernel/combined_member_signature.dart
index 2f22878..641352a 100644
--- a/pkg/front_end/lib/src/fasta/kernel/combined_member_signature.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/combined_member_signature.dart
@@ -332,6 +332,10 @@
                   _coreTypes,
                   _combinedMemberSignatureType,
                   norm(_coreTypes, getMemberType(index)));
+              assert(
+                  _combinedMemberSignatureType != null,
+                  "No combined member signature found for "
+                  "${_mutualSubtypes.values.map((int i) => getMemberType(i))}");
             }
           }
           _neededNnbdTopMerge =
diff --git a/pkg/kernel/lib/src/merge_visitor.dart b/pkg/kernel/lib/src/merge_visitor.dart
index de66d20..f089d75 100644
--- a/pkg/kernel/lib/src/merge_visitor.dart
+++ b/pkg/kernel/lib/src/merge_visitor.dart
@@ -115,9 +115,10 @@
       newNamedParameters[i] = newNamedType;
     }
     TypedefType? newTypedefType;
-    if (a.typedefType != null) {
+    if (a.typedefType != null && b.typedefType != null) {
       newTypedefType = mergeTypes(a.typedefType, b.typedefType) as TypedefType?;
-      if (newTypedefType == null) return null;
+      // If the typedef couldn't be merged we just omit it from the resulting
+      // function type since the typedef type is only informational.
     }
 
     return new FunctionType(newPositionalParameters, newReturnType, nullability,