diff --git a/expression/expr_to_pb_test.go b/expression/expr_to_pb_test.go
index af1e0471260d1..03d80a14d018f 100644
--- a/expression/expr_to_pb_test.go
+++ b/expression/expr_to_pb_test.go
@@ -1178,6 +1178,12 @@ func TestExprPushDownToFlash(t *testing.T) {
 	require.NoError(t, err)
 	exprs = append(exprs, function)
 
+	// CastTimeAsDuration
+	function, err = NewFunction(mock.NewContext(), ast.Cast, types.NewFieldType(mysql.TypeDuration), datetimeColumn)
+	require.NoError(t, err)
+	require.Equal(t, tipb.ScalarFuncSig_CastTimeAsDuration, function.(*ScalarFunction).Function.PbCode())
+	exprs = append(exprs, function)
+
 	pushed, remained = PushDownExprs(sc, exprs, client, kv.TiFlash)
 	require.Len(t, pushed, len(exprs))
 	require.Len(t, remained, 0)
@@ -1185,11 +1191,6 @@ func TestExprPushDownToFlash(t *testing.T) {
 	pushed, remained = PushDownExprsWithExtraInfo(sc, exprs, client, kv.TiFlash, true)
 	require.Len(t, pushed, len(exprs))
 	require.Len(t, remained, 0)
-
-	// CastTimeAsDuration
-	function, err = NewFunction(mock.NewContext(), ast.Cast, types.NewFieldType(mysql.TypeDatetime), durationColumn)
-	require.NoError(t, err)
-	exprs = append(exprs, function)
 }
 
 func TestExprOnlyPushDownToFlash(t *testing.T) {