diff --git a/python/src/com/jetbrains/python/formatter/PyBlock.java b/python/src/com/jetbrains/python/formatter/PyBlock.java
index dc90566d7287..94f967632305 100644
--- a/python/src/com/jetbrains/python/formatter/PyBlock.java
+++ b/python/src/com/jetbrains/python/formatter/PyBlock.java
@@ -86,7 +86,7 @@ public class PyBlock implements Block {
                 dumpSubBlocks();
             }
         }
-        return _subBlocks;
+        return new ArrayList<Block>(_subBlocks);
     }
 
     private List<Block> buildSubBlocks() {