readtextfromfile function error

So I have this readtextfromfile function available to me through a plugin, but I don't understand why it's not working. I have the most basic of the basic interface here just to test its usage in my sandbox, and I'm getting this error: 

Could not display interface. Please check definition and inputs.

Interface Definition: Expression evaluation error at function 'readtextfromfile' [line 2]:

That's it. I'm not sure what the issue is. Does anyone else have more experience with this function? Thank you!

a!localVariables(
  local!extractedText: readtextfromfile(ri!uploadedDoc, true),
  {
    a!sectionLayout(
      label: "Section",
      contents: {
        a!fileUploadField(
          label: "File Upload",
          labelPosition: "ABOVE",
          target: cons!KS_UPLOAD_FILES,
          value: ri!uploadedDoc,
          saveInto: ri!uploadedDoc,
          validations: {}
        )
      }
    )
  }
)