list comparison help?

[Dropkick Punt]

Hi. I have a list of common prefixes:

>>> prefixes = [ "the", "this", "that", "da", "d", "is", "are", "r", "you", "u"]

And I have a string, that I split() into a list.

>>> sentence = "what the blazes is this"
>>> sentence = sentence.split()

Now I want to strip the sentence of all words in the prefix list. 

I tried this method:

>>> for x in prefixes:
...   	  if sentence.index(x):
...             del sentence[sentence.index(x)]


This raises, the error: 

Traceback (most recent call last):
  File "<stdin>", line 3, in ?
ValueError: list.index(x): x not in list

This puzzles me, because if x isn't in the list, the subroutine shouldn't attempt to delete it
from the list, so I'm not sure why it's complaining.

Can anybody explain this to me, &/or show me a better way to do it?