diff --git a/compiler/frontend/src/org/jetbrains/jet/lang/descriptors/FunctionDescriptorImpl.java b/compiler/frontend/src/org/jetbrains/jet/lang/descriptors/FunctionDescriptorImpl.java
index 01e64b9fdf1..f0df94e84be 100644
--- a/compiler/frontend/src/org/jetbrains/jet/lang/descriptors/FunctionDescriptorImpl.java
+++ b/compiler/frontend/src/org/jetbrains/jet/lang/descriptors/FunctionDescriptorImpl.java
@@ -113,6 +113,10 @@ public abstract class FunctionDescriptorImpl extends DeclarationDescriptorImpl i
     }
 
     public void setReturnType(@NotNull JetType unsubstitutedReturnType) {
+        if (this.unsubstitutedReturnType != null) {
+            // TODO: uncomment and fix tests
+            //throw new IllegalStateException("returnType already set");
+        }
         this.unsubstitutedReturnType = unsubstitutedReturnType;
     }