diff --git a/lib/mongo/collection.rb b/lib/mongo/collection.rb
index 71d1c50..e3a31ed 100644
--- a/lib/mongo/collection.rb
+++ b/lib/mongo/collection.rb
@@ -367,7 +367,14 @@ module Mongo
       if spec.is_a?(String) || spec.is_a?(Symbol)
         field_spec[spec.to_s] = 1
       elsif spec.is_a?(Array) && spec.all? {|field| field.is_a?(Array) }
-        spec.each { |f| field_spec[f[0].to_s] = f[1] }
+        spec.each do |f|
+          if [Mongo::ASCENDING, Mongo::DESCENDING, Mongo::GEO2D].include?(f[1])
+            field_spec[f[0].to_s] = f[1]
+          else
+            raise MongoArgumentError, "Invalid index field #{f[1].inspect}; " + 
+              "should be one of Mongo::ASCENDING (1), Mongo::DESCENDING (-1) or Mongo::GEO2D ('2d')."
+          end
+        end
       else
         raise MongoArgumentError, "Invalid index specification #{spec.inspect}; " + 
           "should be either a string, symbol, or an array of arrays."
diff --git a/test/collection_test.rb b/test/collection_test.rb
index f3dbfee..b18e0b8 100644
--- a/test/collection_test.rb
+++ b/test/collection_test.rb
@@ -577,6 +577,12 @@ class TestCollection < Test::Unit::TestCase
       end
     end
 
+    should "enforce proper index types" do
+      assert_raise MongoArgumentError do
+        @collection.create_index([['c', 'blah']])
+      end
+    end
+
     should "generate indexes in the proper order" do
       @collection.expects(:insert_documents) do |sel, coll, safe|
         assert_equal 'b_1_a_1', sel[:name]