[Warning] illegal entry test.v

hi, i have some problems with a testbench when it reads a file with $readmemb("memA.dat",memA). I'm using VCS. It gives me a warning:
[warning] illegal entry found at file memA.dat line 1 while executing $readmem. please ensure that the file has proper entries. The entry is binary numbers(100111001) per line and I suppose it should be a string. So i don't know the cause of the error. If you know, please help me.