find_package cannot locate existing package

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I am attempting to compile a set of packages, yet I get the following error:

CMake Error at /opt/ros/indigo/share/catkin/cmake/catkin_package.cmake:159 (message):
  catkin_package() DEPENDS on 'G2O' which must be find_package()-ed before.
  If it is a catkin package it can be declared as CATKIN_DEPENDS instead
  without find_package()-ing it.
Call Stack (most recent call first):
  /opt/ros/indigo/share/catkin/cmake/catkin_package.cmake:98 (_catkin_package)
  dvo_slam/CMakeLists.txt:24 (catkin_package)

I made sure to source my catkin workspace and confirmed (with rospack) that the package g2o exists. I checked the appropriate CMakeLists.txt, which calls find_package(G2O REQUIRED). I do not understand why I am getting this error.