diff --git a/swash/processing/sws_npz.py b/swash/processing/sws_npz.py
index 8eb9cd0..0afc796 100644
--- a/swash/processing/sws_npz.py
+++ b/swash/processing/sws_npz.py
@@ -3,6 +3,7 @@ import configparser
 import logging
 import pathlib
 
+import numpy as np
 import pandas as pd
 
 from .read_swash import ReadSwash