diff --git a/pkg/analyzer/lib/src/dart/element/type.dart b/pkg/analyzer/lib/src/dart/element/type.dart
index 178000f..7207be9 100644
--- a/pkg/analyzer/lib/src/dart/element/type.dart
+++ b/pkg/analyzer/lib/src/dart/element/type.dart
@@ -1508,9 +1508,11 @@
     //
     List<DartType> jArgs = j.typeArguments;
     List<DartType> jVars = jElement.type.typeArguments;
-    supertype = supertype.substitute2(jArgs, jVars);
-    if (supertype == i) {
-      return true;
+    if (supertype != null) {
+      supertype = supertype.substitute2(jArgs, jVars);
+      if (supertype == i) {
+        return true;
+      }
     }
     //
     // I is listed in the on clause of J.
@@ -2394,6 +2396,13 @@
         return member;
       }
     }
+    for (InterfaceType constraint in targetType.superclassConstraints) {
+      ExecutableElement member = _lookUpMemberInInterfaces(
+          constraint, true, library, visitedInterfaces, getMember);
+      if (member != null) {
+        return member;
+      }
+    }
     for (InterfaceType mixinType in targetType.mixins.reversed) {
       ExecutableElement member = _lookUpMemberInInterfaces(
           mixinType, true, library, visitedInterfaces, getMember);
diff --git a/pkg/analyzer/test/src/dart/resolution/mixin_test.dart b/pkg/analyzer/test/src/dart/resolution/mixin_test.dart
index 0375fc2..0361250 100644
--- a/pkg/analyzer/test/src/dart/resolution/mixin_test.dart
+++ b/pkg/analyzer/test/src/dart/resolution/mixin_test.dart
@@ -1579,6 +1579,30 @@
     ]);
   }
 
+  test_isMoreSpecificThan() async {
+    addTestFile(r'''
+mixin M {}
+''');
+    await resolveTestFile();
+    assertNoTestErrors();
+
+    var element = findElement.mixin('M');
+    var type = element.type;
+    expect(type.isMoreSpecificThan(intType), isFalse);
+  }
+
+  test_lookUpMemberInInterfaces_Object() async {
+    addTestFile(r'''
+class Foo {}
+
+mixin UnhappyMixin on Foo {
+  String toString() => '$runtimeType';
+}
+''');
+    await resolveTestFile();
+    assertNoTestErrors();
+  }
+
   test_metadata() async {
     addTestFile(r'''
 const a = 0;