Using: set_global_assignment -name QIP_FILE in .tcl script fails

Title says it all really.

I have a setup that requires the use of environment variables when setting these kind of assignments. Most seem to work, but when it comes to QIP_FILE assignments in a tcl script, Quartus errors.

Using an explicit path in the .qsf project file works. Is there any way of using environment variables in a .qsf file, or am I doing something wrong in my .tcl script?

The line I am using is:

set_global_assignment -name QIP_FILE $::env(MY_VAR_PATH)/my/path/my.qip

Any help on this would be great,

Thanks in advance!