diff --git a/src/main/java/com/android/tools/r8/graph/AppInfoWithClassHierarchy.java b/src/main/java/com/android/tools/r8/graph/AppInfoWithClassHierarchy.java
index 87a2343..de380e8 100644
--- a/src/main/java/com/android/tools/r8/graph/AppInfoWithClassHierarchy.java
+++ b/src/main/java/com/android/tools/r8/graph/AppInfoWithClassHierarchy.java
@@ -36,12 +36,16 @@
   public boolean isSubtype(DexType subtype, DexType supertype) {
     assert subtype != null;
     assert supertype != null;
+    assert subtype.isClassType();
+    assert supertype.isClassType();
     return subtype == supertype || isStrictSubtypeOf(subtype, supertype);
   }
 
   public boolean isStrictSubtypeOf(DexType subtype, DexType supertype) {
     assert subtype != null;
     assert supertype != null;
+    assert subtype.isClassType();
+    assert supertype.isClassType();
     if (subtype == supertype) {
       return false;
     }
diff --git a/src/main/java/com/android/tools/r8/shaking/MainDexListBuilder.java b/src/main/java/com/android/tools/r8/shaking/MainDexListBuilder.java
index 15be9af..bfefefa 100644
--- a/src/main/java/com/android/tools/r8/shaking/MainDexListBuilder.java
+++ b/src/main/java/com/android/tools/r8/shaking/MainDexListBuilder.java
@@ -101,12 +101,14 @@
           DexProto proto = method.method.proto;
           if (proto.parameters.isEmpty()) {
             DexType valueType = proto.returnType.toBaseType(appInfo.dexItemFactory());
-            if (isEnum(valueType)) {
-              value = true;
-              break;
-            } else if (isAnnotation(valueType) && isAnnotationWithEnum(valueType)) {
-              value = true;
-              break;
+            if (valueType.isClassType()) {
+              if (isEnum(valueType)) {
+                value = true;
+                break;
+              } else if (isAnnotation(valueType) && isAnnotationWithEnum(valueType)) {
+                value = true;
+                break;
+              }
             }
           }
         }