Invalid value at 'requests[0].update_cells.range' (type.googleapis.com/google.apps.sheets.v4.GridRange)

Question

I'm trying to set the background of a single specific cell in column C via the NodeJS Google Sheets library. However, I am receiving this error

errors: [
    {
      message: `Invalid value at 'requests[0].update_cells.range' (type.googleapis.com/google.apps.sheets.v4.GridRange), "'Question of the day'!C2"`,
      reason: 'invalid'
    }
  ]

This is my method call

      sheets.spreadsheets.batchUpdate({
        auth: this._token,
        spreadsheetId: this._spreadsheet,
        resource: {
          requests: [{
            updateCells: {
              range: '\'Question of the day\'!C' + (index + 1),
              fields: 'userEnteredFormat',
              rows: [{
                values: [{
                  userEnteredFormat: {
                    backgroundColor: {
                      red: 1,
                      green: 0,
                      blue: 0
                    }
                  }
                }]
              }]
            }
          }]
        }
      })

The auth is correct as well as the spreadsheet. I tested the range the error throws in Google Sheets and it's a valid and working range.

I'm not sure why this is happening