I need help with vLookup. Excel changes my array range.

I have my price list with 100 product numbers and matching prices. I have
another list of products, that I'm trying fill in the prices for.

Col A - product numbers
Col B - prices
Col C - a list of product numbers I'm trying to get prices for
Col D - where I want to put the correct price for the product in column C

In D1, I put in the formula
vlookup(C1,A1:b100,2, true)
and then I copy the formula to the rest of column D

The problem is, when Excel copies the formula, it changes the lookup range.
Example, in D45, the formula actually looks like
vlookup(c45,a45:b100,2,true)

Somehow it changed the lookup range to start at a45.

Should I be using a different function?