I cannot find anything regarding that issue on stackoverflow either.

Code-39 doesn't seem to support any lower case letters.

Is there a way I can .toUpperCase in that code?

From Wikipedia: "The Code 39 specification defines 43 characters, consisting of uppercase letters (A through Z), numeric digits (0 through 9) and a number of special characters (-, ., $, /, +, %, and space). An additional character (denoted '*') is used for both start and stop delimiters."

Thank you, very helpful. I'm learning FreeMarker and I fixed It. You saved me hours.