This integral is missing an integration domain

Let's consider I want to implement a model with e.g. a source term like

...
b = q * phi * dx
...

Now, if its value is set to zero (q=0), I get the following error from ufl:

Traceback (most recent call last):
  File "example.py", line 123, in <module>
    b = q * phi * dx
  File "/usr/lib/python3/dist-packages/ufl/measure.py", line 441, in __rmul__
    error("This integral is missing an integration domain.")
  File "/usr/lib/python3/dist-packages/ufl/log.py", line 172, in error
    raise self._exception_type(self._format_raw(*message))
ufl.log.UFLException: This integral is missing an integration domain.

I understand that the integral is evaluated to zero, but I think this is not very viable.

Often I want to have such terms to be able to add a (source) term at some other time, but its annoying to comment this line everytime if I change my models parameters.

Is there a way to change/overcome this?