Are there line breaks or spaces between each token in the file? I had a similar issue and turned out that linebreaks are not shown in debug mode, so it was hard to find...
Just to be sure, check if len(letterText) returns 1.
Also try changing encoding of the text file. You can press Save As in Notepad, save with a different encoding method to a folder and then re-import the file to the project.
If nothing helps, could you share your capx?