Unable to access jarfile when PLANTUML_JAR contains spaces on Windows

[rose-a]

If PLANTUML_JAR contains a path with spaces in it, compiling results in an error (Example path: C:\Program Files (x86)\PlantUML\plantuml.jar):

Error: Unable to access jarfile C:\Program

I tried it out running the java cmd on the command line, same result there. If you put quotes around the environment variable everything works fine.