invalid character 'i' looking for beginning of value

Question

I have created API in go which takes input id and in response gives user data. Now when I post data to API either Postman or using form, it gives me error invalid character 'i' looking for beginning of value I tried to search and implemented various solution but nothing works.

Router

r.Use(func(c *gin.Context) {
        // add header Access-Control-Allow-Origin
        c.Writer.Header().Set("Content-Type", "application/json")
        c.Writer.Header().Set("Access-Control-Allow-Origin", "*")
        c.Writer.Header().Set("Access-Control-Max-Age", "86400")
        c.Writer.Header().Set("Access-Control-Allow-Methods", "POST, GET, PUT, DELETE, UPDATE")
        c.Writer.Header().Set("Access-Control-Allow-Headers", "Content-Type, Content-Length, Accept-Encoding, X-CSRF-Token, Authorization, X-Max")
        c.Writer.Header().Set("Access-Control-Allow-Credentials", "true")

        if c.Request.Method == "OPTIONS" {
            c.AbortWithStatus(200)
        } else {
            c.Next()
        }
    })
v1 := r.Group("/api/v1")
v1.Use(middlewares.UserMiddlewares())
{
    v1.POST("user-list", apiControllerV1.UserList)
}

Api Call to function

func UserList(c *gin.Context) {
    var userService v1s.UserService
    err := json.NewDecoder(c.Request.Body).Decode(&userService.User)
    fmt.Println(err)
    if err != nil {
        u.Respond(c.Writer, u.Message(1, "Invalid request"))
        return
    }

}

Struct

type Model struct {
    ID        uint       `gorm:"primary_key" json:"id,omitempty"`
    CreatedAt time.Time  `gorm:"not null" json:"created_at" sql:"DEFAULT:CURRENT_TIMESTAMP"`
    UpdatedAt time.Time  `gorm:"not null" json:"updated_at" sql:"DEFAULT:CURRENT_TIMESTAMP"`
    DeletedAt *time.Time `sql:"index" json:"deleted_at,omitempty"`
}
type User struct {
    Model
    Name  string `gorm:"type:varchar(50)" json:"name" validate:"required"`
    Email string `gorm:"type:varchar(50)" json:"email" validate:"required,email"`
}

Posting data

id:1 or {id:1}