Error: getenv cannot be interpreted at compile time

[Paul via Digitalmars-d-learn]

I'd like to create a Terminal using terminal.d and make it 
available across several source files (as a global rather than 
having to pass it around) but when I define it like this in 
global scope:

Terminal Screen = Terminal(ConsoleOutputType.cellular);
I get this error:

Error: getenv cannot be interpreted at compile time, because it 
has no available source code

I don't understand the error and Google doesn't help - can it be 
fixed or am I just using the wrong approach?

TIA

Paul