diff --git a/aosstower/level_b1/nc.py b/aosstower/level_b1/nc.py
index 84e36b81dcb39f6fc3665f7dc33aff879019955e..e170050deb37de9345ee1af745c0317d347c6315 100644
--- a/aosstower/level_b1/nc.py
+++ b/aosstower/level_b1/nc.py
@@ -37,7 +37,8 @@ def _get_data(input_files):
 
 def get_data(input_files):
     frame = pd.DataFrame(_get_data(input_files))
-    frame = frame.set_index('stamp').mask(frame == -99999.).fillna(value=np.nan)
+    frame = frame.set_index('stamp')
+    frame = frame.mask(frame == -99999.).fillna(value=np.nan)
     return frame