Running loop to make all fields uppercase... getting an error in the console.

Good evening, I have created a script to spell check a very large list of medications and cancers for our company.

I have created a loop to make all the fields lowercase, before spell checking the document.... However, the fields are being made to lowercase, and then I am prompted the error: 

"InvalidSetError: Set not possible, invalid or unknown.

Field.value:22:Batch undefined:Exec"

Here is my code for the loops:

for (i = 0; i < this.numFields; i++) {
var AllFields = this.getField(this.getNthFieldName(i));
AllFields.value = AllFields.valueAsString.toLowerCase();
}
spell.addWord(items); 
spell.check();
for (i = 0; i < this.numFields; i++) {
var AllFields = this.getField(this.getNthFieldName(i));
AllFields.value = AllFields.valueAsString.toUpperCase();
}

Any help would be greatly appreciated!