diff --git a/src/backend/utils/adt/windowfuncs.c b/src/backend/utils/adt/windowfuncs.c
index b87a624fb2..f4ff060930 100644
--- a/src/backend/utils/adt/windowfuncs.c
+++ b/src/backend/utils/adt/windowfuncs.c
@@ -696,15 +696,16 @@ window_nth_value(PG_FUNCTION_ARGS)
 	int32		nth;
 
 	nth = DatumGetInt32(WinGetFuncArgCurrent(winobj, 1, &isnull));
-	if (isnull)
-		PG_RETURN_NULL();
-	const_offset = get_fn_expr_arg_stable(fcinfo->flinfo, 1);
-
 	if (nth <= 0)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_ARGUMENT_FOR_NTH_VALUE),
 				 errmsg("argument of nth_value must be greater than zero")));
 
+	if (isnull)
+		PG_RETURN_NULL();
+
+	const_offset = get_fn_expr_arg_stable(fcinfo->flinfo, 1);
+
 	result = WinGetFuncArgInFrame(winobj, 0,
 								  nth - 1, WINDOW_SEEK_HEAD, const_offset,
 								  &isnull, NULL);