The BSON encoder does not handle struct properties that can be nil. If it does encounter a nil struct property it returns an error when it should encode that value into a BSON Null.

To fix this:

• Update underlyingVal to check if the value is nil when it's Kind is either a reflect.Ptr or a reflect.Interface. If it is, don't return the underlying Elem, instead return the pointer or interface.
• In encodeMap, encodeSlice, encodeSliceAsArray, and encodeStruct, after calling underlyingVal, if the kind is an interface, map, pointer, or slice, and it is nil, return a BSON Null type.