error in altera-modelsim simulation

Hi am trying to simulate a design in altera-modelsim for a simple design am just trying to observe the internal signal of a design in testbench using $display or $monitor but it throws a error at that line is there any particular sytax in altera-modelsim?

can you please help me out

$monitor("data:%d\n",test_tb.top.Interface.start);

here start is the internal signal to be observed