Error generating objects in a for loop

When I run the next code, I get this error:

>(Pdb) 
>Traceback (most recent call last):
> File "simulacion.py", line 203, in <module>
>    env.run( until = 120*10000 )
>  File "/usr/local/lib/python2.7/dist-packages/http://ift.tt/1IGZVpW", line 137, in run
    self.step()
>  File "/usr/local/lib/python2.7/dist-packages/http://ift.tt/1IGZVpW", line 229, in step
>    raise exc
>TypeError: not all arguments converted during string formatting
>Exception AttributeError: "'NoneType' object has no attribute 'path'" in ><function _remove at 0xb7254bc4> ignored

The code:

import pdb
import simpy
import random 
import itertools
import sys

while(True):
    ...
    r = random.uniform(0,1)
    t = random.expovariate(300)
    yield env.timeout(t)
    ....

I found the error, just when I run the yield env.timeout(t) line