### Mertens Function Error

I am new to Sage, so I suppose the answer to this is ridiculously obvious, but please go easy on me!

I have an error when I try to get Mertens function as an output yet: I know the summation works for other functions; and I know the Moebius function is defined on the range. So what's going on?

This is the input and output:

var('m');
var('j');
f=moebius(j)
sum(f,j,0,m)

TypeError Traceback (most recent call last)
<ipython-input-1-29d7fd7804be> in <module>()
1 var('m');
2 var('j');
----> 3 f=moebius(j)
4 sum(f,j,Integer(0),m)

/home/sc_serv/sage/local/lib/python2.7/site-packages/sage/rings/arith.pyc in __call__(self, n)
3831 n = -n
3832 F = factor(n)
-> 3833 for _, e in F:
3834 if e >= 2:
3835 return 0

TypeError: 'sage.symbolic.expression.Expression' object is not iterable