diff --git a/fastimage.gemspec b/fastimage.gemspec
index 3eb0e97..0e7e085 100644
--- a/fastimage.gemspec
+++ b/fastimage.gemspec
@@ -20,7 +20,7 @@ Gem::Specification.new do |s|
   s.require_paths = ["lib"]
   s.rubygems_version = %q{1.3.6}
   s.summary = %q{FastImage - Image info fast}
-  s.add_development_dependency 'fakeweb', '~> 1.3'
+  s.add_development_dependency 'fakeweb-fi', '~> 1.3'
   # Note rake 11 drops support for ruby 1.9.2
   s.add_development_dependency('rake', '~> 10.5')
   s.add_development_dependency('rdoc')
diff --git a/lib/fastimage.rb b/lib/fastimage.rb
index a17c69d..c00f965 100644
--- a/lib/fastimage.rb
+++ b/lib/fastimage.rb
@@ -483,7 +483,12 @@ class FastImage
     when 0x89.chr + "P"
       :png
     when "II", "MM"
-      :tiff
+      case @stream.peek(11)[8..10]
+      when "APC", "CR\002"
+        nil  # do not recognise CRW or CR2 as tiff
+      else
+        :tiff
+      end
     when '8B'
       :psd
     when "\0\0"
diff --git a/test/fixtures/a.CR2 b/test/fixtures/a.CR2
new file mode 100644
index 0000000..220b074
Binary files /dev/null and b/test/fixtures/a.CR2 differ
diff --git a/test/fixtures/a.CRW b/test/fixtures/a.CRW
new file mode 100644
index 0000000..95953c0
Binary files /dev/null and b/test/fixtures/a.CRW differ
diff --git a/test/test.rb b/test/test.rb
index 30a5f2d..51efd28 100644
--- a/test/test.rb
+++ b/test/test.rb
@@ -41,7 +41,9 @@ GoodFixtures = {
 BadFixtures = [
   "faulty.jpg",
   "test_rgb.ct",
-  "test.xml"
+  "test.xml",
+  "a.CR2",
+  "a.CRW"
 ]
 # man.ico courtesy of http://www.iconseeker.com/search-icon/artists-valley-sample/business-man-blue.html
 # test_rgb.ct courtesy of http://fileformats.archiveteam.org/wiki/Scitex_CT
@@ -409,4 +411,13 @@ class FastImageTest < Test::Unit::TestCase
   def test_should_return_content_length_for_data_uri_images
     assert_equal DataUriImageContentLength, FastImage.new(DataUriImage).content_length
   end
+  
+  def test_canon_raw_formats_are_not_recognised_as_tiff
+    assert_raises(FastImage::UnknownImageType) do
+      FastImage.size(TestUrl + "a.CR2", :raise_on_failure=>true)
+    end
+    assert_raises(FastImage::UnknownImageType) do
+      FastImage.size(TestUrl + "a.CRW", :raise_on_failure=>true)
+    end
+  end
 end