Disabling single signals in expression coverage?

I am running expression coverage on a design. There are many ATPG multiplexers implemented in RTL, such as

wire signal = aptg_mode ? something : something_else;

Expression coverage complains that, since atpg_mode is always 0 in RTL simulations, the ternary expression is only 50% tested. Is there something I can do about it? Like, a pragma that tells the coverage scoring that atpg_mode is considered to be constant '0'?