Serverless Invalid Variable Reference Syntax for Variable ([\S\S]+?)

I am getting the following error when doing a serverless deploy

Invalid variable reference syntax for variable ([\s\S]+?). You can only reference env vars, options, & files. You can check our docs for more info.

I think it is caused by the following line in my serverless.yml (in the environment block of the provider block):

variableSyntax: "\\${{([\\s\\S]+?)}}"

Why is this invalid? Or is the error actually elsewhere, and if it is, how can I see what line in what file is causing the error?