diff --git a/compiler/frontend/src/org/jetbrains/jet/lang/types/lang/KotlinBuiltIns.java b/compiler/frontend/src/org/jetbrains/jet/lang/types/lang/KotlinBuiltIns.java
index f36c32c4ba3..30d4f6804b0 100644
--- a/compiler/frontend/src/org/jetbrains/jet/lang/types/lang/KotlinBuiltIns.java
+++ b/compiler/frontend/src/org/jetbrains/jet/lang/types/lang/KotlinBuiltIns.java
@@ -136,7 +136,7 @@ public class KotlinBuiltIns {
     private final ResolveSession resolveSession;
     private final ModuleDescriptor builtInsModule;
 
-    private ImmutableSet<ClassDescriptor> nonPhysicalClasses;
+    private volatile ImmutableSet<ClassDescriptor> nonPhysicalClasses;
 
     private final ImmutableSet<ClassDescriptor> functionClassesSet;