cast protobuf enum return 0

[zibber2015]

version: 1.5 1.3

test.proto

enum N {
   DEFAULT = 0;
   ONE =  1;
}

message Test {
    N  num  = 1;
}

main.go

a := &pb.Test{ Num:  123 }

cast.toInt32(a)   //return 0
int32(a)   //return 123