diff --git a/geomagio/TimeseriesUtility.py b/geomagio/TimeseriesUtility.py
index 7fe2488633282c6dcb5e02c958953552a318fe01..e82ab8d70a357045e8c8e1a3b022d82b370b695f 100644
--- a/geomagio/TimeseriesUtility.py
+++ b/geomagio/TimeseriesUtility.py
@@ -223,7 +223,9 @@ def merge_streams(*streams):
             # 1 = do not interpolate
             interpolation_samples=0,
             # 1 = when there is overlap, use data from trace with last endtime
-            method=1)
+            method=1,
+            # np.nan = work-around for (problematic) intermediate masked arrays
+            filled_value=numpy.nan)
 
     # convert back to NaN filled array
     merged = unmask_stream(split)