Problem with SYS_mount

Hello,

I need to mount a loopback file system. I'm using SYS_mount, but it 
always fails with ENOTDIR. Am I doing something silly? Here's the 
offending line:

if(syscall(SYS_mount,
   "/home/sam/mnt/", "/home/sam/myfile.img", "-o loop" ))
         perror("Mount failure/ ");

Thanks.