diff --git a/umsgpack.py b/umsgpack.py
index c0647e0..ddc5151 100644
--- a/umsgpack.py
+++ b/umsgpack.py
@@ -123,6 +123,13 @@ def __str__(self):
         s += ")"
         return s
 
+    def __hash__(self):
+        """
+        Allow to hash extensions, thus allowing them as map keys.
+        """
+        return hash((self.type, self.data))
+
+
 class InvalidString(bytes):
     """Subclass of bytes to hold invalid UTF-8 strings."""
     pass