diff --git a/python/edu/learn-python/src/com/jetbrains/python/edu/StudyDocumentListener.java b/python/edu/learn-python/src/com/jetbrains/python/edu/StudyDocumentListener.java
index f72135d7f181..9fdcf704a29b 100644
--- a/python/edu/learn-python/src/com/jetbrains/python/edu/StudyDocumentListener.java
+++ b/python/edu/learn-python/src/com/jetbrains/python/edu/StudyDocumentListener.java
@@ -51,7 +51,7 @@ public class StudyDocumentListener extends DocumentAdapter {
       int change = event.getNewLength() - event.getOldLength();
       if (myTaskWindow != null) {
         int newLength = myTaskWindow.getLength() + change;
-        myTaskWindow.setLength(newLength);
+        myTaskWindow.setLength(newLength <= 0 ? 0 : newLength);
       }
       int newEnd = offset + event.getNewLength();
       int newLine = document.getLineNumber(newEnd);
diff --git a/python/edu/learn-python/src/com/jetbrains/python/edu/course/TaskFile.java b/python/edu/learn-python/src/com/jetbrains/python/edu/course/TaskFile.java
index ed4f2bfe80da..aa6a5becddb3 100644
--- a/python/edu/learn-python/src/com/jetbrains/python/edu/course/TaskFile.java
+++ b/python/edu/learn-python/src/com/jetbrains/python/edu/course/TaskFile.java
@@ -112,7 +112,8 @@ public class TaskFile implements Stateful{
     for (TaskWindow tw : taskWindows) {
       if (tw.getLine() <= line) {
         int twStartOffset = tw.getRealStartOffset(document);
-        int twEndOffset = twStartOffset + tw.getLength();
+        final int length = tw.getLength() > 0 ? tw.getLength() : 0;
+        int twEndOffset = twStartOffset + length;
         if (twStartOffset <= offset && offset <= twEndOffset) {
           return tw;
         }