diff --git a/swash/processing/read_swash.py b/swash/processing/read_swash.py
index 9287e30..aa43c4b 100644
--- a/swash/processing/read_swash.py
+++ b/swash/processing/read_swash.py
@@ -7,7 +7,7 @@ class ReadSwash:
         self._n_t = None
         self._t = None
         self._x = None
-        self._data = None
+        self._data = {}
 
     @classmethod
     def read_nohead(cls, path):