diff --git a/lib/mongo.rb b/lib/mongo.rb
index 9998ebd..a967c47 100644
--- a/lib/mongo.rb
+++ b/lib/mongo.rb
@@ -11,7 +11,7 @@ begin
     # Need this for running test with and without c ext in Ruby 1.9.
     raise LoadError if ENV['TEST_MODE'] && !ENV['C_EXT']
     require 'mongo_ext/cbson'
-    raise LoadError if CBson::VERSION != Mongo::VERSION
+    raise LoadError unless defined?(CBson::VERSION) && CBson::VERSION == Mongo::VERSION
     require 'mongo/util/bson_c'
     BSON            = BSON_C
   rescue LoadError