NullReferenceException when sub-field of obj contains null

[reinux]

This is OK:

type Test = {
  inside: {| zbc: obj option |}
}

"""
  { "inside": { "zbc": null } }
"""
|> Json.deserializeEx<Test> (JsonConfig.create(allowUntyped = true))

This results in an NRE:

type Test = {
  inside: obj option
}

"""
  { "inside": { "zbc": null } }
"""
|> Json.deserializeEx<Test> (JsonConfig.create(allowUntyped = true))

Stack trace:

System.NullReferenceException: Object reference not set to an instance of an object.
at FSharp.Json.Core.deserializeNonOption@322(JsonConfig config, JsonPath path, Type t, JsonField jsonField, JsonValue jvalue)
at FSharp.Json.Core.mapping@1(JsonConfig config, JsonPath path, Type itemValueType, String field0, JsonValue field1)
at FSharp.Json.Core.deserializeMap@399(JsonConfig config, JsonPath path, Type t, JsonValue jvalue)
at FSharp.Json.Core.deserializeNonOption@322(JsonConfig config, JsonPath path, Type t, JsonField jsonField, JsonValue jvalue)
at FSharp.Json.Core.deserializeUnwrapOption@376(JsonConfig config, JsonPath path, Type t, JsonField jsonField, FSharpOption`1 jvalue)
at FSharp.Json.Core.deserializeRecord@463(JsonConfig config, JsonPath path, Type t, JsonValue jvalue)
at FSharp.Json.Json.deserializeEx[T](JsonConfig config, String json)
at <StartupCode$FSI_0055>.$FSI_0055.main@() in /home/rei/source/HAInterface/HAInterface/Script1.fsx:line 64
at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)
Stopped due to error