GraphQL mutation with Input type doesn't pass argument to resolver

I am trying to execute a mutation taking an input type object as parameter but the parameter is not passed to the resolver

Below are my typeDefs.ts and resolver.ts files:

typeDefs.ts

type Mutation {
  registerUser(input: UserRegistrationInput): AuthPayload
}

input UserRegistrationInput {
  email: String
  password: String
}

type AuthPayload {
  token: String
  message: String
}

resolver.ts

Mutation: {
  registerUser: (_, args) => {
    console.log(args)
  }
}

When I execute this mutation in the GraphQL playground with the query variables, console.log() echo an empty object {}

Mutation

// Mutation
mutation RegisterUser($input: UserRegistrationInput) {
  registerUser(input: $input) {
    message
    token
  }
}

// Query variables
{
  "input": {
    "email": "john.doe@example.com",
    "password": "123456"
  }
}

However if the mutation takes different parameters instead of an input type, it works. console.log() echo the object { "email": "john.doe@example.com", "password": "123456" }

typeDefs.ts

type Mutation {
  registerUser(email: String, password: String): AuthPayload
}

Mutation

// Mutation
mutation RegisterUser($email: String, $password: String) {
  registerUser(email: $email, password: $password) {
    message
    token
  }
}

// Query variables
{
  "email": "john.doe@example.com",
  "password": "123456"
}

I can't figure out why in case of input type the arguments are not passed to the resolver.