From fcbd15dae86ea03a67e1fb047dbeae36a96a1dda Mon Sep 17 00:00:00 2001
From: qiuyesuifeng <cuiqiu.bupt@gmail.com>
Date: Mon, 4 Jan 2016 10:34:41 +0800
Subject: [PATCH] optimizer: address comment.

---
 optimizer/evaluator/evaluator.go | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/optimizer/evaluator/evaluator.go b/optimizer/evaluator/evaluator.go
index 64c726f2c3..d8d87909d2 100644
--- a/optimizer/evaluator/evaluator.go
+++ b/optimizer/evaluator/evaluator.go
@@ -55,9 +55,12 @@ func Eval(ctx context.Context, expr ast.ExprNode) (interface{}, error) {
 // EvalBool evalueates an expression to a boolean value.
 func EvalBool(ctx context.Context, expr ast.ExprNode) (bool, error) {
 	val, err := Eval(ctx, expr)
-	if err != nil || val == nil {
+	if err != nil {
 		return false, errors.Trace(err)
 	}
+	if val == nil {
+		return false, nil
+	}
 
 	i, err := types.ToBool(val)
 	if err != nil {