When the directory containing the ui form files for your project has a space in its path (.../ProjectName/UI Forms/, for example), the following error is generated during the qmake process (Qt is installed in C:\Program Files (x86)):

'C:\Program' is not recognized as an internal or external command,
operable program or batch file.

When the directory containing the ui form files doesn't have a space (.../ProjectName/UI_Forms/, for example), this error does not appear. I have tried surrounding the individual form file path entries with quotes to no avail. I have also tried system_path, shell_path, clean_path, absolute_path, etc. to get a valid path for each individual form. Regardless of the syntax, this error still shows up.

Note: These form files are passed to uic via the FORMS environment variable as usual (and are not being generated manually).