diff --git a/lib/xmet/skew_t.py b/lib/xmet/skew_t.py
index 01ef6a5..134cd8a 100644
--- a/lib/xmet/skew_t.py
+++ b/lib/xmet/skew_t.py
@@ -195,7 +195,7 @@ class SkewTGraph():
         first = True
 
         for sample in sounding.samples:
-            if sample.pressure < 0 or sample.pressure is None:
+            if sample.pressure is None or sample.pressure < 0:
                 continue
 
             if sample.pressure < PRESSURE_MIN: