diff --git a/lib/fakefs/file.rb b/lib/fakefs/file.rb
index a14c6b9..db14ef3 100644
--- a/lib/fakefs/file.rb
+++ b/lib/fakefs/file.rb
@@ -100,7 +100,9 @@ module FakeFS
     end
 
     attr_reader :path
-    def initialize(path, mode = nil, perm = nil)
+    def initialize(path, mode = READ_ONLY, perm = nil)
+      check_mode(mode)
+
       @path = path
       @mode = mode
       @file = FileSystem.find(path)
@@ -146,5 +148,11 @@ module FakeFS
     def read_only?
       @mode == READ_ONLY
     end
+
+    def check_mode(mode)
+      if !MODES.include?(mode)
+        raise ArgumentError, "illegal access mode #{mode}"
+      end
+    end
   end
 end
diff --git a/test/fakefs_test.rb b/test/fakefs_test.rb
index 5b9d69d..22b08cc 100644
--- a/test/fakefs_test.rb
+++ b/test/fakefs_test.rb
@@ -106,6 +106,14 @@ class FakeFSTest < Test::Unit::TestCase
     end
   end
 
+  def test_file_opens_in_invalid_mode
+    FileUtils.touch("foo")
+
+    assert_raises(ArgumentError) do
+      File.open("foo", "an_illegal_mode")
+    end
+  end
+
   def test_can_read_files_once_written
     path = '/path/to/file.txt'
     File.open(path, 'w') do |f|