VHDL仿真时出现warning

VHDL仿真时出现warning如下：# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 0  Instance: /comp_module1_unit_testbench

请问什么原因，仿真波形显示并没有错误。