using operator assignments without getting recursion error

Does anyone use the /= assignment operator?

I am trying to do a

while error  > error_tol do

sequence of ops,

# update error from last loop
# simple example of assignment test 

error /= 2;    #to simulate decreasing error each loop.  Real equation on RHS is error(i) = error(i-1) + comparison of last iterates.

# Real operator assignment I'd like to use is error += comparison

end do;

the divide / keeps applying as the single divide and a long line under the variable before I can type =.   This happens in both 1-D and 2-D.   The "Operator Assignments" help page doesn't have a lot of help on syntax problems using these. 

Thanks,
Bill