Serverless-pseudo-parameters plugin not working

Serverless Framework
1

Hello,
I’m trying to use the serverless-pseudo-parameters plugin in order to access the AWS::AccountId. I’m attempting to use this in the Resources section of my serverless.yml file.
> resources:

    Resources:
         MyResource:
               .....
            SourceIdentifier:arn:aws:lambda:${opt:region}:#{AWS::AccountId}:function:${self:service}-${opt:stage}-myfunctionname

When I try to deploy I get this error:

Type Error ---------------------------------------------
TypeError: Cannot convert undefined or null to object
at Function.keys ()
at replaceChildNodes (/Users/sandbox/node_modules/serverless-pseudo-parameters/lib/index.js:71:14)
at /Users/sandbox/node_modules/serverless-pseudo-parameters/lib/index.js:34:7
at Array.forEach ()

I am using:
serverless-pseudo-parameters: “2.5.0”
Node Version: 12.14.1
Framework Version: 1.61.1

Any help with this would be appreciated.