Error when running test-invoke-authorizer: tokenSignature

I am attempting to create a custom authorizer for IoT and am following this guide:
https://aws.amazon.com/blogs/security/how-to-use-your-own-identity-and-access-management-systems-to-control-access-to-aws-iot-resources/

When I attempt to test my authorizer with this command:
aws iot test-invoke-authorizer --authorizer-name <authorizer_name> --token allow --token-signature <token_signature>

I get the response:
An error occurred (InvalidRequestException) when calling the TestInvokeAuthorizer operation: 1 validation error detected: Value '<signature here>' at 'tokenSignature' failed to satisfy constraint: Member must satisfy regular expression pattern: [A-Za-z0-9_/]_={0,2}

I am testing it in Centos, and have created a bash script like this:
#!/bin/bash
sig=cat token.sign.b64
aws iot test-invoke-authorizer --authorizer-name MyAuthorizer --token allow --token-signature "$sig"

The token.sign.b64 is created as per the instructions in the guide.

How do I get it to authorize correctly?