diff --git a/pkg/entity/constraint.go b/pkg/entity/constraint.go
index 64c6f725..a9db2b48 100644
--- a/pkg/entity/constraint.go
+++ b/pkg/entity/constraint.go
@@ -69,7 +69,7 @@ func (c *Constraint) toExprStr() (string, error) {
 		return "", fmt.Errorf("not supported operator: %s", c.Operator)
 	}
 
-	return fmt.Sprintf("{%s} %s %s", c.Property, o, c.Value), nil
+	return fmt.Sprintf("({%s} %s %s)", c.Property, o, c.Value), nil
 }
 
 // Validate validates Constraint
diff --git a/pkg/handler/eval_test.go b/pkg/handler/eval_test.go
index 53e96109..b51e169c 100644
--- a/pkg/handler/eval_test.go
+++ b/pkg/handler/eval_test.go
@@ -116,19 +116,26 @@ func TestEvalFlag(t *testing.T) {
 				Value:     `"CA"`,
 			},
 			{
-				Model:     gorm.Model{ID: 500},
+				Model:     gorm.Model{ID: 501},
 				SegmentID: 200,
 				Property:  "state",
 				Operator:  models.ConstraintOperatorEQ,
 				Value:     `{dl_state}`,
 			},
+			{
+				Model:     gorm.Model{ID: 502},
+				SegmentID: 200,
+				Property:  "rate",
+				Operator:  models.ConstraintOperatorGT,
+				Value:     `1000`,
+			},
 		}
 		f.PrepareEvaluation()
 		cache := &EvalCache{mapCache: map[uint]*entity.Flag{100: &f}}
 		defer gostub.StubFunc(&GetEvalCache, cache).Reset()
 		result := evalFlag(models.EvalContext{
 			EnableDebug:   true,
-			EntityContext: map[string]interface{}{"dl_state": "CA", "state": "CA"},
+			EntityContext: map[string]interface{}{"dl_state": "CA", "state": "CA", "rate": 2000},
 			EntityID:      util.StringPtr("entityID1"),
 			EntityType:    util.StringPtr("entityType1"),
 			FlagID:        util.Int64Ptr(int64(100)),