Compiler assertion on incorrect number of arguments to primitive function

Here is a test file:

$ cat test.sac 
int main ()
{
  return _add_SxS_ (1);
}

Here is what I get when calling sac2c:

/tmp|=> sac2c test.sac 
Internal compiler error
Assertion "TYgetProductSize( args) == 2" failed at /tmp/sac2c/src/libsac2c/typecheck/ct_prf.c:2476 -- ari_op_SxV called with incorrect number of arguments
Please file a bug at: http://bugs.sac-home.org

This should be an error, not a compiler assertion.