diff --git a/src/servers/app/decorator/DefaultDecorator.cpp b/src/servers/app/decorator/DefaultDecorator.cpp
index 0dc34cdd5d..7b07782169 100644
--- a/src/servers/app/decorator/DefaultDecorator.cpp
+++ b/src/servers/app/decorator/DefaultDecorator.cpp
@@ -1247,7 +1247,8 @@ DefaultDecorator::_ResizeBy(BPoint offset, BRegion* dirty)
 	if (fTitleBarRect.IsValid()) {
 		if (fTabList.CountItems() > 1) {
 			_DoTabLayout();
-			dirty->Include(fTitleBarRect);
+			if (dirty != NULL)
+				dirty->Include(fTitleBarRect);
 			return;
 		}