diff --git a/core/api_coupon.go b/core/api_coupon.go
index f475a02..74fd4dd 100644
--- a/core/api_coupon.go
+++ b/core/api_coupon.go
@@ -261,8 +261,8 @@ func listAllCouponNames(mongoClient gocommon.MongoClient, w http.ResponseWriter,
 }
 
 func useCoupon(mongoClient gocommon.MongoClient, w http.ResponseWriter, r *http.Request) {
-	acc, _ := gocommon.ReadStringFormValue(r.Form, "accid")
-	if len(acc) == 0 {
+	acc, ok := gocommon.ReadObjectIDFormValue(r.Form, "accid")
+	if !ok || acc.IsZero() {
 		w.WriteHeader(http.StatusBadRequest)
 		return
 	}