diff --git a/pmda/custom.py b/pmda/custom.py
index 7fda4c19..6a35329d 100644
--- a/pmda/custom.py
+++ b/pmda/custom.py
@@ -100,7 +100,7 @@ def _single_frame(self, ts, atomgroups):
         return self.function(*args, **self.kwargs)
 
     def _conclude(self):
-        self.results = np.hstack(self._results)
+        self.results = np.vstack(self._results)
 
 
 def analysis_class(function):