diff --git a/mavextra.py b/mavextra.py
index 90e81d26d..a959c28ff 100644
--- a/mavextra.py
+++ b/mavextra.py
@@ -271,6 +271,8 @@ def second_derivative_9(var, key):
 
 def lowpass(var, key, factor):
     '''a simple lowpass filter'''
+    if isnan(var):
+        return None
     global lowpass_data
     if not key in lowpass_data:
         lowpass_data[key] = var