diff --git a/s/Wimp02 b/s/Wimp02
index 633866310e5694a03faa77910ef2effad16b9a34..c4792bc83d79ba5524d0f6ba25425c3ba22eb8e8 100644
--- a/s/Wimp02
+++ b/s/Wimp02
@@ -4030,6 +4030,7 @@ SWIWimp_SetExtent
 
 checkextent
         Push    "cx0,cy0,cx1,cy1,LR"
+        BL      checkredrawhandle               ; ensure no task switch for validation string reads
 ;
         BL      minwindowx
         Pull    "cx0,cy0,cx1,cy1,PC",VS