diff --git a/src/evalwindow.c b/src/evalwindow.c
index c7a9f79..534a047 100644
--- a/src/evalwindow.c
+++ b/src/evalwindow.c
@@ -810,7 +810,8 @@
     targetwin = find_win_by_nr_or_id(&argvars[1]);
 
     if (wp == NULL || targetwin == NULL || wp == targetwin
-	    || !win_valid(wp) || !win_valid(targetwin))
+	    || !win_valid(wp) || !win_valid(targetwin)
+	    || win_valid_popup(wp) || win_valid_popup(targetwin))
     {
         emsg(_(e_invalwindow));
 	rettv->vval.v_number = -1;