diff --git a/studyprocessor.py b/studyprocessor.py
index 12672fa..83ef311 100644
--- a/studyprocessor.py
+++ b/studyprocessor.py
@@ -136,7 +136,7 @@ class StudyProcessor(Aggregator, HDF5Container):
             series[run] = []
             for i, num in enumerate(self.nums[run]):
                 series[run].append(getter(run, num, arg=arg))
-            series[run] = np.array(series[run])
+            series[run] = np.array(series[run], dtype=float)
         return series
 
     def _compare(self, getter, use_num=True):