diff --git a/core/descriptors/src/org/jetbrains/jet/lang/types/TypeConstructorImpl.java b/core/descriptors/src/org/jetbrains/jet/lang/types/TypeConstructorImpl.java
index 96801af7625..8668ed5a16c 100644
--- a/core/descriptors/src/org/jetbrains/jet/lang/types/TypeConstructorImpl.java
+++ b/core/descriptors/src/org/jetbrains/jet/lang/types/TypeConstructorImpl.java
@@ -30,7 +30,7 @@ import java.util.List;
 
 public class TypeConstructorImpl extends AnnotatedImpl implements TypeConstructor {
     private final List<TypeParameterDescriptor> parameters;
-    private Collection<JetType> supertypes;
+    private final Collection<JetType> supertypes;
     private final String debugName;
     private final boolean isFinal;