I am checking the code coverage for one of my Module's testbench, but the expression coverage is appearing as "n/a" , why is that so ? there are if-else & assignments in the RTL... how do I enable it ?

I am checking the code coverage for one of my Module's testbench, but the expression coverage is appearing as "n/a" , why is that so ? there are if-else & assignments in the RTL... how do I enable it ?