diff --git a/platform/analysis-impl/src/com/intellij/codeInsight/daemon/impl/FileStatusMap.java b/platform/analysis-impl/src/com/intellij/codeInsight/daemon/impl/FileStatusMap.java
index 7acc45ecbdf3..439bef719247 100644
--- a/platform/analysis-impl/src/com/intellij/codeInsight/daemon/impl/FileStatusMap.java
+++ b/platform/analysis-impl/src/com/intellij/codeInsight/daemon/impl/FileStatusMap.java
@@ -176,7 +176,7 @@ public class FileStatusMap implements Disposable {
 
   private void assertAllowModifications() {
     try {
-      assert myAllowDirt;
+      assert myAllowDirt : "PSI/document/model changes are not allowed during highlighting";
     }
     finally {
       myAllowDirt = true; //give next test a chance