problem with isnan

[Charles Hixson via Digitalmars-d-learn]

The line:

assert    (isnan (c.curActivation), "cell has unexpected curActivation: 
%s".format(c.curActivation));

throws the exception:

core.exception.AssertError at cell.d(285): cell has unexpected 
curActivation: nan

and I've looked at it backwards and forwards and don't understand why.  
It's *supposed* to be nan, and the assert message reports that it is, 
but it should pass the assert test, not throw an assertion.  What am I 
doing wrong?