diff --git a/PIL/Image.py b/PIL/Image.py
index ed1d10907..c643e24d9 100644
--- a/PIL/Image.py
+++ b/PIL/Image.py
@@ -1715,7 +1715,10 @@ class Image(object):
         if not format:
             if ext not in EXTENSION:
                 init()
-            format = EXTENSION[ext]
+            try:
+                format = EXTENSION[ext]
+            except KeyError:
+                raise ValueError('unknown file extension: {}'.format(ext))
 
         if format.upper() not in SAVE:
             init()
diff --git a/Tests/test_image.py b/Tests/test_image.py
index 1bdbd3960..6aefc4f73 100644
--- a/Tests/test_image.py
+++ b/Tests/test_image.py
@@ -87,6 +87,11 @@ class TestImage(PillowTestCase):
             reloaded = Image.open(fp)
             self.assert_image_similar(im, reloaded, 20)
 
+    def test_unknown_extension(self):
+        im = hopper()
+        temp_file = self.tempfile("temp.unknown")
+        self.assertRaises(ValueError, lambda: im.save(temp_file))
+
     def test_internals(self):
 
         im = Image.new("L", (100, 100))