diff --git a/pyModeS/decoder/common.py b/pyModeS/decoder/common.py
index 4225568..99384c4 100644
--- a/pyModeS/decoder/common.py
+++ b/pyModeS/decoder/common.py
@@ -205,8 +205,8 @@ def altcode(msg):
         int: altitude in ft
     """
 
-    if df(msg) not in [4, 20]:
-        raise RuntimeError("Message must be Downlink Format 4 or 20.")
+    if df(msg) not in [0, 4, 16, 20]:
+        raise RuntimeError("Message must be Downlink Format 0, 4, 16, or 20.")
 
     # Altitude code, bit 20-32
     mbin = hex2bin(msg)