diff --git a/HISTORY.rst b/HISTORY.rst
index bbc2350..cac3736 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -3,6 +3,9 @@
 History
 -------
 
+* Provide a more useful error message when ``MODE_MMAP_EXT`` is requested but
+  the C extension is not available.
+
 1.2.3 (2017-01-11)
 ++++++++++++++++++
 
diff --git a/maxminddb/__init__.py b/maxminddb/__init__.py
index 600f998..0f3d810 100644
--- a/maxminddb/__init__.py
+++ b/maxminddb/__init__.py
@@ -27,8 +27,11 @@ def open_database(database, mode=MODE_AUTO):
             * MODE_AUTO - tries MODE_MMAP_EXT, MODE_MMAP, MODE_FILE in that
                           order. Default mode.
     """
-    if (mode == MODE_AUTO and maxminddb.extension and
-            hasattr(maxminddb.extension, 'Reader')) or mode == MODE_MMAP_EXT:
+    has_extension = maxminddb.extension and hasattr(maxminddb.extension, 'Reader')
+    if (mode == MODE_AUTO and has_extension) or mode == MODE_MMAP_EXT:
+        if not has_extension:
+            raise ValueError(
+                "MODE_MMAP_EXT requires the maxminddb.extension module to be available")
         return maxminddb.extension.Reader(database)
     elif mode in (MODE_AUTO, MODE_MMAP, MODE_FILE, MODE_MEMORY):
         return maxminddb.reader.Reader(database, mode)
diff --git a/tests/reader_test.py b/tests/reader_test.py
index 2eae039..a561adb 100644
--- a/tests/reader_test.py
+++ b/tests/reader_test.py
@@ -94,6 +94,14 @@ def test_ipv6_address_in_ipv4_database(self):
             reader.get('2001::')
         reader.close()
 
+    def test_no_extension_exception(self):
+        real_extension = maxminddb.extension
+        maxminddb.extension = None
+        with self.assertRaisesRegex(ValueError, 'MODE_MMAP_EXT requires the maxminddb.extension module to be available'):
+            open_database(
+            'tests/data/test-data/MaxMind-DB-test-decoder.mmdb', MODE_MMAP_EXT)
+        maxminddb.extension = real_extension
+
     def test_broken_database(self):
         reader = open_database('tests/data/test-data/'
                                'GeoIP2-City-Test-Broken-Double-Format.mmdb',