diff --git a/platform/editor-ui-ex/src/com/intellij/openapi/editor/ex/util/LayeredLexerEditorHighlighter.java b/platform/editor-ui-ex/src/com/intellij/openapi/editor/ex/util/LayeredLexerEditorHighlighter.java
index 1328c862c46e..8719c0735840 100644
--- a/platform/editor-ui-ex/src/com/intellij/openapi/editor/ex/util/LayeredLexerEditorHighlighter.java
+++ b/platform/editor-ui-ex/src/com/intellij/openapi/editor/ex/util/LayeredLexerEditorHighlighter.java
@@ -16,6 +16,8 @@
 
 package com.intellij.openapi.editor.ex.util;
 
+import com.intellij.openapi.diagnostic.Attachment;
+import com.intellij.openapi.diagnostic.Logger;
 import com.intellij.openapi.editor.Document;
 import com.intellij.openapi.editor.RangeMarker;
 import com.intellij.openapi.editor.colors.EditorColorsScheme;
@@ -26,10 +28,12 @@ import com.intellij.openapi.editor.highlighter.HighlighterClient;
 import com.intellij.openapi.editor.highlighter.HighlighterIterator;
 import com.intellij.openapi.editor.impl.DocumentImpl;
 import com.intellij.openapi.editor.markup.TextAttributes;
+import com.intellij.openapi.fileEditor.FileDocumentManager;
 import com.intellij.openapi.fileTypes.SyntaxHighlighter;
 import com.intellij.openapi.fileTypes.SyntaxHighlighterBase;
 import com.intellij.openapi.project.Project;
 import com.intellij.openapi.util.Comparing;
+import com.intellij.openapi.vfs.VirtualFile;
 import com.intellij.psi.tree.IElementType;
 import com.intellij.util.containers.FactoryMap;
 import com.intellij.util.containers.IntArrayList;
@@ -45,6 +49,7 @@ import java.util.*;
  * @author max
  */
 public class LayeredLexerEditorHighlighter extends LexerEditorHighlighter {
+  private static final Logger LOG = Logger.getInstance("#com.intellij.openapi.editor.ex.util.LayeredLexerEditorHighlighter");
   private final Map<IElementType, LayerDescriptor> myTokensToLayer = new HashMap<>();
   private final Map<LayerDescriptor, Mapper> myLayerBuffers = new HashMap<>();
 
@@ -107,11 +112,20 @@ public class LayeredLexerEditorHighlighter extends LexerEditorHighlighter {
         final int len = lengths.get(i);
         start += mySeparator.length();
         final int globalIndex = index2Global.get(i);
-        assert getSegments().myRanges[globalIndex] == null : myText;
+        checkNull(type, getSegments().myRanges[globalIndex]);
         getSegments().myRanges[globalIndex] = new MappedRange(mapper, document.createRangeMarker(start, start + len), type);
         start += len;
       }
     }
+
+    private void checkNull(IElementType type, MappedRange range) {
+      if (range != null) {
+        Document mainDocument = getDocument();
+        VirtualFile file = mainDocument == null ? null : FileDocumentManager.getInstance().getFile(mainDocument);
+        LOG.error("Expected null range on " + type + ", found " + range + "; highlighter=" + getSyntaxHighlighter(),
+                  new Attachment(file != null ? file.getName() : "editorText.txt", myText.toString()));
+      }
+    }
   }
 
   @Override
@@ -456,6 +470,11 @@ public class LayeredLexerEditorHighlighter extends LexerEditorHighlighter {
       this.outerToken = outerToken;
       assert mapper.doc == range.getDocument();
     }
+
+    @Override
+    public String toString() {
+      return "MappedRange{range=" + range + ", outerToken=" + outerToken + '}';
+    }
   }
 
   @Nullable