Unknown type ‘TGuid’

I upgraded from Inno Setup 5.3.10 to 5.4.0 and tried to build a project that has previously built without any issues.

Got this error instead:

Error on line 223 in setup.iss: Column 10:
Unknown type 'TGuid'
Compile aborted.

A while back I had stopped using the Non Unicode Inno Setup and started using Unicode Inno Setup instead. When I upgraded to 5.4.0 I accidentally reverted to the Non Unicode version. Once I removed ispack-5.4.0.exe and installed ispack-5.4.0-unicode.exe the error went away.