ChargeTransactionNullArgument error on Checkout transaction request

Every time I try to call the Checkout transaction endpoint through our application I get a ChargeTransactionNullArgument error. I am already adding the authorization_token I get from the checkout process. This is my request body:
{
"amount": 100,
"authorization_token": <<transactionToken I get when checkout is complete>>,
"application_fee": 0
}

And this is the response I get:

[{"message":"Transaction is required.","error_name":"ChargeTransactionNullArgument","error_code":14,"raw_message":"Transaction is required."}]

When sending the same request on Blackbaud console,on the checkout transaction documentation page, it works just fine.