Problem with sign / sgn and .n()

edit

I have an urgent problem with the sign function. I have done a symbolic calculation and want to evaluate the result numerically. The symbolic terms have a sgn() function within, which gives me an error when I try to evaluate the expression numerically. Please find the error below. I do not have the slightest ideas why sage can not evaluate that (simple) expression:

sign(1050*cos(61075634/181449129) + 900).n()

How can i solve that problem?

Thanks a lot in advance,

Best Markus

Error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "_sage_input_60.py", line 10, in <module>
    exec compile(u'open("___code___.py","w").write("# -*- coding: utf-8 -*-\\n" + .....

execfile(os.path.abspath("___code___.py"))
  File "", line 1, in <module>

  File "/tmp/tmp4W3s9l/___code___.py", line 4, in <module>
    exec compile(u'qwer.n()
  File "", line 1, in <module>

  File "expression.pyx", line 4053, in sage.symbolic.expression.Expression._numerical_approx (sage/symbolic/expression.cpp:17894)
TypeError: cannot evaluate symbolic expression numerically

edit by kcrisman: Here's a minimal example:

sage: M = sgn(cos(3/2))
sage: M.n()

It doesn't suffice to do sgn(cos(1)), because that turns out to be 1.