@ai asked me to open up an issue after he helped me to understand that an error I was encountering was being caused by setting a declaration value with a number instead of a string:

const paddingTop = postcss.decl({ prop: 'top', value: 0 })

This would in turn throw the following error: TypeError: decl.value.indexOf is not a function

I believe the idea we discussed would be to automatically cast number values to strings to improve DX in a case like this rather than throwing an error.