diff --git a/src/evalwindow.c b/src/evalwindow.c
index 98c7ff3..bc59251 100644
--- a/src/evalwindow.c
+++ b/src/evalwindow.c
@@ -889,6 +889,11 @@
     wp = find_win_by_nr_or_id(&argvars[0]);
     if (wp == NULL || win_valid_popup(wp))
 	return;
+    if (!win_valid(wp))
+    {
+	emsg(_(e_cannot_resize_window_in_another_tab_page));
+	return;
+    }
 
     offset = (int)tv_get_number(&argvars[1]);
     win_drag_vsep_line(wp, offset);