GraphQL client not reading certain query variables

Question

I have a database that consists of the following 3 entries:

{
  "username": "negger",
  "firstName": "Arnold"
},
{
  "username": "jonsnow",
  "firstName": "Jon"
},
{
  "username": "tonystark",
  "firstName": "Tony"
}

In my GraphQL Playground (https://dev.schandillia.com/graphql), you can run the following query:

{
  users (where: {username: "jonsnow"}) {
    username
    firstName
  }
}

And it'll return the matching data as you'd expect:

{
  "data": {
    "users": [
      {
        "username": "jonsnow",
        "firstName": "Jon"
      }
    ]
  }
}

However, if you try using query variables in order to dynamically lookup entries by username, the where clause is simply ignored! Here's the dynamic query I'm running:

query users($username: String!) {
  users(where: {username: $username}) {
    username
    firstName
  }
}

And here's the query variable definition:

{
  "username": "jonsnow"
}

My question is, what sorcery is this? Why wouldn't the playground client respect query variables even when the value being passed exists!