diff --git a/src/evalfunc.c b/src/evalfunc.c
index eb082b7..c18ab31 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -5800,7 +5800,7 @@
 
     if (argvars[0].v_type != VAR_UNKNOWN)
     {
-	wparg = win_id2wp(argvars);
+	wparg = win_id2wp(tv_get_number(&argvars[0]));
 	if (wparg == NULL)
 	    return;
     }