diff --git a/core.py b/core.py
index ecc803d5011d94a64204ce981eda5bb6ff9e073b..f3712adabe63e4f97c251c0d016a1cb872694bd6 100644
--- a/core.py
+++ b/core.py
@@ -92,7 +92,7 @@ def runner(meta, flagger, data, flags=None, nodata=np.nan):
 
             try:
                 dchunk, ffchunk = flagDispatch(func_name,
-                                               dchunk, fchunk, varname,
+                                               dchunk, fchunk.copy(), varname,
                                                flagger, nodata=nodata,
                                                **flag_params)
             except NameError: