Handling missing combinations of factors in R

So, I have a data frame with two factors and one numeric variable like so:

>D
f1 f2 v1 
1   A  23
2   A  45
2   B  27
     .
     .
     .

so the levels of f1 are 1 and 2 and the levels of f2 are A and B. Here's the thing, there is no value inputted for when f1=1 and f2=B (that is D$V1[D$f1=1 & D$f2=B] isn't there) in reality this should be zero.

In my actual data frame I have 11 levels of f1 and close to 150 levels of f2 and I need to create an observation with v1=0 for every combination of f1 and f2 that is missing from my data frame.

How would I go about doing this?

Thanks in advance,

Ian