diff --git a/compiler/frontend/src/org/jetbrains/jet/lang/descriptors/MutableClassDescriptorLite.java b/compiler/frontend/src/org/jetbrains/jet/lang/descriptors/MutableClassDescriptorLite.java
index de433513b11..c7e45fd090e 100644
--- a/compiler/frontend/src/org/jetbrains/jet/lang/descriptors/MutableClassDescriptorLite.java
+++ b/compiler/frontend/src/org/jetbrains/jet/lang/descriptors/MutableClassDescriptorLite.java
@@ -282,6 +282,9 @@ public class MutableClassDescriptorLite extends MutableDeclarationDescriptor imp
 
     public void addSupertype(@NotNull JetType supertype) {
         if (!ErrorUtils.isErrorType(supertype)) {
+            if (!(supertype.getConstructor().getDeclarationDescriptor() instanceof ClassDescriptor)) {
+                throw new IllegalStateException();
+            }
             supertypes.add(supertype);
         }
     }