Error Evaluating UI Expression - Expression evaluation error at function fn!content_appian_internal [line 12]: Invalid Content ID

Hi,

Scenario:

I am trying to create an interface which accepts an uploaded excel file. When the user hits 'Submit' on the interface it starts a process model that passes the excel file into an excel parsing smart service and returns that data in the form of a list/dictionary of a CDT model.

Before I get to any of this, though, I seem to be getting the following error when I try to debug/run my process model:

Error Evaluating UI Expression Expression evaluation error in rule 'bulkuploadedit' at function a!fileUploadField [line 17]: Error in a!forEach() expression during iteration 1: Expression evaluation error at function fn!content_appian_internal [line 12]: Invalid Content ID

Here is the code for my interface:

a!formLayout(
  label: "Bulk Edit Invoice Numbers of Events",
  contents: {
    a!sectionLayout(
      label: "Upload File",
      contents: {
        a!fileUploadField(
          label: "Please upload your excel spreadsheet here.",
          labelPosition: "ABOVE",
          helptooltip: "Note: only EventId and ClientInvoiceNumber are accepted column headers.",
          target: cons!EXCEL_UPLOAD_FOLDER_POINTER,
          maxSelections: 1,
          value: ri!documentId,
          saveInto: a!save(ri!documentId,save!value),
          validations: {},
          buttonstyle: "PRIMARY"
        )
      }
    )
  },
  buttons: a!buttonLayout(
    primaryButtons: {
      a!buttonWidget(
        label: "Submit",
        saveInto: ri!documentId,
        style: "PRIMARY",
        submit: true
      )
    },
    secondaryButtons: {
      a!buttonWidget(
        label: "Cancel",
        value: true,
        saveInto: ri!cancel,
        style: "NORMAL",
        submit: true,
        validate: false
      )
    }
  )
)

Does anyone know whats going wrong here?

Thanks.