diff --git a/platform/core-impl/src/com/intellij/openapi/editor/impl/LineSet.java b/platform/core-impl/src/com/intellij/openapi/editor/impl/LineSet.java
index 1d3e1dfa4482..973ec2af3aaa 100644
--- a/platform/core-impl/src/com/intellij/openapi/editor/impl/LineSet.java
+++ b/platform/core-impl/src/com/intellij/openapi/editor/impl/LineSet.java
@@ -184,10 +184,15 @@ public class LineSet{
   }
 
   LineSet clearModificationFlags(int startLine, int endLine) {
-    if (startLine == endLine) return this;
+    if (startLine > endLine) {
+      throw new IllegalArgumentException("endLine < startLine: " + endLine + " < " + startLine + "; lineCount: " + getLineCount());
+    }
     checkLineIndex(startLine);
     checkLineIndex(endLine - 1);
 
+    if (isLastEmptyLine(endLine - 1)) endLine--;
+    if (startLine >= endLine) return this;
+
     byte[] flags = myFlags.clone();
     for (int i = startLine; i < endLine; i++) {
       flags[i] &= ~MODIFIED_MASK;