I have the following function in a google app script running on a google sheet.

function GetRecords(username) {
  var ss= SpreadsheetApp.getActiveSpreadsheet();
  var displaySheet = ss.getSheetByName("DISPLAY SHEETS");
  var getLastRow =  displaySheet.getLastRow();
  var gradeArray = [];
  for(var i = 2; i <= getLastRow; i++)
  {
    var gradeSheetName = displaySheet.getRange(i, 1).getValue();
    var gradeSheet = ss.getSheetByName(gradeSheetName);
    var getLastRowGrade =  gradeSheet.getLastRow();
    for(var y = 2; y <= getLastRowGrade; y++)
    {
      if(gradeSheet.getRange(y, 1).getValue().toUpperCase() == username.toUpperCase())
      {
        var grade = gradeSheet.getRange(y, 2).getValue();
        gradeArray.push([username.toUpperCase(), gradeSheetName, grade]);
      }
    }
  }
  return gradeArray;  
}

It was working fine up until yesterday, where it now returns the following type error:

TypeError: gradeSheet.getRange(...).getValue(...).toUpperCase is not a function
    at GetRecords(Code:17:47)

Any ideas on how to fix this?

Thanks