Go json Marshaller panics with "call of reflect.Value.Int on zero Value"

I am trying to unmarshal some data in a complex struct with multi-level nesting (thus, not copying the struct here). However, when the code went live, we started getting the following panic in rare occasions (difficult to quantify but perhaps 1 in 1000 cases).

Below is the stack trace of the issue:

panic: reflect: call of reflect.Value.Int on zero Value [recovered]
        panic: reflect: call of reflect.Value.Int on zero Value

    goroutine 568428 [running]:
    encoding/json.(*encodeState).marshal.func1(0xc0081d5c70)
            /usr/local/go/src/encoding/json/encode.go:305 +0x9a
    panic(0x13968c0, 0xc005c2f540)
            /usr/local/go/src/runtime/panic.go:679 +0x1b2
    reflect.Value.Int(...)
            /usr/local/go/src/reflect/value.go:986
    encoding/json.intEncoder(0xc0074b5dc0, 0x0, 0x0, 0x0, 0xc0038b0100)
            /usr/local/go/src/encoding/json/encode.go:522 +0x1d4
    encoding/json.mapEncoder.encode(0x1608760, 0xc0074b5dc0, 0x1385c20, 0xc00119f5e0, 0x195, 0x100)
            /usr/local/go/src/encoding/json/encode.go:706 +0x351
    encoding/json.structEncoder.encode(0xc0002ce600, 0x19, 0x21, 0xc00027ede0, 0xc0074b5dc0, 0x156c820, 0xc00119f510, 0x199, 0x520100)
            /usr/local/go/src/encoding/json/encode.go:664 +0x306
    encoding/json.ptrEncoder.encode(0xc00027ee10, 0xc0074b5dc0, 0x1397e80, 0xc00119f510, 0x16, 0x1390100)
            /usr/local/go/src/encoding/json/encode.go:810 +0xb1
    encoding/json.(*encodeState).reflectValue(0xc0074b5dc0, 0x1397e80, 0xc00119f510, 0x16, 0x100)
            /usr/local/go/src/encoding/json/encode.go:337 +0x82
    encoding/json.(*encodeState).marshal(0xc0074b5dc0, 0x1397e80, 0xc00119f510, 0xc003da0100, 0x0, 0x0)
            /usr/local/go/src/encoding/json/encode.go:309 +0x10b
    encoding/json.Marshal(0x1397e80, 0xc00119f510, 0x40be53, 0x13c4160, 0x1397e80, 0x1, 0x7fa9d5d348b0)
            /usr/local/go/src/encoding/json/encode.go:161 +0x52
    reingames.com/rm/user.(*userCore).MarshalBinary(0xc00119f510, 0x1397e80, 0xc00119f510, 0x7fa9d5d348b0, 0xc00119f510, 0x1)
            /home/shivam/goprojects/rmgs/go/user/profile.go:180 +0x37

I have no clue where to start debugging this. Has anyone experienced something similar? Quick question - could a race condition (marshalling a pointer while it is being updated) cause this panic?

Thanks

Yes, a race could cause almost anything, including this. If you have any tests to speak of, run them with
go test -race. If not, or if that doesn't turn anything up, you might consider canarying a build with
go build -race, but see the race detector documentation for caveats about performance and memory usage in that case. You may just want to inspect the code yourself for races.

One way to start debugging is : catch the panic with a recover() instruction, and log information about the context of the error.

This wouldn't catch a race condition "in the act", but it would certainly help you understand more things about your error (does it occur always on the same elements ? is the element still in a valid state ? etc ...)

Recently, we also have this problem, you can check whether it is a coroutine in the code,Is there a right slice operate。Slice relies on array implementation. When the underlying array has sufficient capacity, the append operation is not a read-only operation, and elements are directly added to the free position of the array. Therefore, when a multi-coroutine performs an append operation on a global slice, it will operate on the same underlying data, causing read-write conflicts. Causes the exception of Go json Marshaller.