REG: Error from qmake on previously working .pro file

The following .pro file sets a variable to the contents of an environment variable and checks its contents, which is a pretty common pattern in my projects to enable overriding paths etc. with environment variables.

TESTVAR = $$(NON_EXISTENT_ENVVAR)
isEmpty($$TESTVAR): message("Environment variable missing")

In Qt 5.9 (for instance, I only tested 5.9 and 5.12), this prints out "Environment variable missing" as supposed to, but in Qt 5.12 qmake will exit with an error saying isEmpty(var) requires one argument.