Unable to convert interface type to custom type in Go? [duplicate]

Question

I am trying to type convert an interface to a struct

Why?

This is because I don't know before hand what the type of payload i will get, so the default type is interface{}

BUT, using a switch case, I need to convert the interface type to parse payload using a specific function:

/*********************************************
TYPES: 
 EventPayload   is of type    -> interface{}
 NewUserPayload is of type    -> struct {
    Username string `json:"username"`
    UserId   string `json:"userId"`
 }
*********************************************/

switch payload.EventName {
  case events.NEW_USER:
    c.newUserHandler(payload.EventPayload.(core.NewUserPayload)) // 👈 error
  case events.DIRECT_MESSAGE:
    c.directMessageHandler(payload.EventPayload.(core.DirectMessagePayload))
}

Error message:

interface conversion: interface {} is map[string]interface {}, not core.NewUserPayload