diff --git a/lib/mongoid/extensions/objectid/conversions.rb b/lib/mongoid/extensions/objectid/conversions.rb
index 71b279aaae..06e346dc9d 100644
--- a/lib/mongoid/extensions/objectid/conversions.rb
+++ b/lib/mongoid/extensions/objectid/conversions.rb
@@ -5,7 +5,7 @@ module ObjectID #:nodoc:
       module Conversions #:nodoc:
         def set(value)
           if value.is_a?(::String)
-            BSON::ObjectID.from_string(value)
+            BSON::ObjectID.from_string(value) unless value.blank?
           else
             value
           end
diff --git a/spec/integration/mongoid/extensions_spec.rb b/spec/integration/mongoid/extensions_spec.rb
index b39bc5b0e9..69e35ce56b 100644
--- a/spec/integration/mongoid/extensions_spec.rb
+++ b/spec/integration/mongoid/extensions_spec.rb
@@ -22,32 +22,35 @@
 
   context "setting association foreign keys" do
 
-    context "when value is a string" do
+    let(:game) { Game.new }
+    let(:person) { Person.create(:ssn => "555555555555555") }
 
-      before do
-        @game = Game.new
-        @person = Person.create(:ssn => "555555555555555")
+    context "when value is an empty string" do
+
+      it "should set the foreign key to nil" do
+        game.person_id = ""
+        game.save
+        game.reload.person_id.should be_nil
       end
 
+    end
+
+    context "when value is a populated string" do
+
       it "should set the foreign key as ObjectID" do
-        @game.person_id = @person.id.to_s
-        @game.save
-        @game.reload.person_id.should == @person.id
+        game.person_id = person.id.to_s
+        game.save
+        game.reload.person_id.should == person.id
       end
 
     end
 
     context "when value is a ObjectID" do
 
-      before do
-        @game = Game.new
-        @person = Person.create(:ssn => "555555555555555")
-      end
-
       it "should keep the the foreign key as ObjectID" do
-        @game.person_id = @person.id
-        @game.save
-        @game.reload.person_id.should == @person.id
+        game.person_id = person.id
+        game.save
+        game.reload.person_id.should == person.id
       end
 
     end
diff --git a/spec/unit/mongoid/extensions/objectid/conversions_spec.rb b/spec/unit/mongoid/extensions/objectid/conversions_spec.rb
index c0ec8b77a6..3a4aa35e2a 100644
--- a/spec/unit/mongoid/extensions/objectid/conversions_spec.rb
+++ b/spec/unit/mongoid/extensions/objectid/conversions_spec.rb
@@ -2,13 +2,7 @@
 
 describe Mongoid::Extensions::ObjectID::Conversions do
 
-  let(:object_id) do
-    BSON::ObjectID.new
-  end
-
-  let(:object_id_string) do
-    "4c52c439931a90ab29000003"
-  end
+  let(:object_id) { BSON::ObjectID.new }
 
   describe "#get" do
 
@@ -18,18 +12,26 @@
 
   end
 
-  describe "#set with ObjectID" do
+  describe "#set" do
 
-    it "returns self" do
-      BSON::ObjectID.set(object_id).should == object_id
-    end
+    let(:object_id_string) { "4c52c439931a90ab29000003" }
 
-  end
+    context "with a blank string" do
+      it "returns nil" do
+        BSON::ObjectID.set("").should be_nil
+      end
+    end
 
-  describe "#set with String" do
+    context "with a populated string" do
+      it "returns ObjectID" do
+        BSON::ObjectID.set(object_id_string).should == BSON::ObjectID.from_string(object_id_string)
+      end
+    end
 
-    it "returns ObjectID" do
-      BSON::ObjectID.set(object_id_string).should == BSON::ObjectID.from_string(object_id_string)
+    context "with an ObjectID" do
+      it "returns self" do
+        BSON::ObjectID.set(object_id).should == object_id
+      end
     end
 
   end