This is already the current behaviour.
```
In [3]: sin(x) + cos(y)
Out[3]: sin(x) + cos(y)

In [4]: _.series()
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
/home/prasoon/Documents/code/workspace/sympy/<ipython-input-4-6d01b3355af6> in <module>()
----> 1 _.series()

/home/prasoon/Documents/code/workspace/sympy/sympy/core/expr.pyc in series(self, x, x0, n, dir)
   2297             syms = self.atoms(C.Symbol)
   2298             if len(syms) > 1:
-> 2299                 raise ValueError('x must be given for multivariate functions.')
   2300             x = syms.pop()
   2301 

ValueError: x must be given for multivariate functions.

```