GraphQL: Variable 'input' has coerced Null value for NonNull type 'CustomerSearchInput!'

Question

I'm trying to make an API call to search for a Braintree customer (by the customer's ID).

https://graphql.braintreepayments.com/guides/making_api_calls/

I've set up the API call with a graphql query that looks like this.

{
"query": "query Search($input: CustomerSearchInput!) { 
search { 
  customers(input: $input) { 
    pageInfo { 
      hasNextPage 
      startCursor 
      endCursor 
    }, 
    edges { 
      node { 
        id 
        } 
      } 
    } 
  } 
}",
  "variables": {
    "input": {
      "id": {
        "is": "7492836219"
      }
    }
  }
}

The query part of this always works but for some reason, it doesn't seem to detect any variables. I keep getting an error that says the following.

{"errors":[{"message":"Variable 'input' has coerced Null value for NonNull type 'CustomerSearchInput!'","locations":[{"line":1,"column":14}]}],"extensions":{"requestId":"ab399854-21ba-47a3-bf70-e41c3097dd21"}}

It seems like it's not recognizing the "variables" section of this object but I have no idea why not. Does anyone know why I keep getting this error?