diff --git a/mini3di/encoder.py b/mini3di/encoder.py
index 7496d3a..2867626 100644
--- a/mini3di/encoder.py
+++ b/mini3di/encoder.py
@@ -230,7 +230,7 @@ def _create_nan_mask(
         """
         mask_ca = numpy.isnan(ca).max(axis=1)
         mask_n = numpy.isnan(n).max(axis=1)
-        mask_c = numpy.isnan(n).max(axis=1)
+        mask_c = numpy.isnan(c).max(axis=1)
         return (mask_ca | mask_n | mask_c).repeat(3).reshape(-1, 3)
 
     def encode_atoms(