raise ArgumentError when DC[nil] is invoked or DC.orm = nil.

lib/database_cleaner/base.rb

@@ -11,8 +11,8 @@ module DatabaseCleaner
       [orm, db] <=> [other.orm, other.db]
     end
 
-    def initialize(desired_orm = nil, opts = {})
-      self.orm = desired_orm
+    def initialize(orm = :null, opts = {})
+      self.orm = orm
       self.db = opts[:connection] || opts[:model] if opts.has_key?(:connection) || opts.has_key?(:model)
       Safeguard.new.run
     end
@@ -44,8 +44,9 @@ module DatabaseCleaner
 
     attr_reader :orm
 
-    def orm=(desired_orm)
-      @orm = desired_orm && desired_orm.to_sym
+    def orm= orm
+      raise ArgumentError if orm.nil?
+      @orm = orm.to_sym
     end
 
     extend Forwardable

lib/database_cleaner/configuration.rb

@@ -4,13 +4,12 @@ require 'forwardable'
 
 module DatabaseCleaner
 
-  class NoORMDetected < StandardError; end
   class UnknownStrategySpecified < ArgumentError; end
 
   class Cleaners < Hash
     # FIXME this method conflates creation with lookup... both a command and a query. yuck.
     def [](orm, opts = {})
-      raise NoORMDetected unless orm
+      raise ArgumentError if orm.nil?
       fetch([orm, opts]) { add_cleaner(orm, opts) }
     end 
 

spec/database_cleaner/base_spec.rb

@@ -46,15 +46,14 @@ module DatabaseCleaner
           expect(cleaner.orm).to eq :mongoid
         end
 
-        it "should default to nil" do
+        it "should default to :null" do
           cleaner = Base.new
-          expect(cleaner.orm).to be_nil
+          expect(cleaner.orm).to eq :null
         end
 
-        it "can handle being set to nil" do
+        it "raises ArgumentError when explicitly set to nil" do
           cleaner = Base.new
-          cleaner.orm = nil
-          expect(cleaner.orm).to be_nil
+          expect { cleaner.orm = nil }.to raise_error(ArgumentError)
         end
       end
     end

spec/database_cleaner/configuration_spec.rb

@@ -12,8 +12,8 @@ RSpec.describe DatabaseCleaner::Configuration do
   subject(:config) { described_class.new }
 
   context "orm specification" do
-    it "should not accept unrecognised orms" do
-      expect { config[nil] }.to raise_error(DatabaseCleaner::NoORMDetected)
+    it "should not accept nil orms" do
+      expect { config[nil] }.to raise_error(ArgumentError)
     end
 
     it "should accept :active_record" do