diff --git a/wfl/fit/error.py b/wfl/fit/error.py
index 14d97285..e64a5c51 100755
--- a/wfl/fit/error.py
+++ b/wfl/fit/error.py
@@ -154,9 +154,9 @@ def _reshape_normalize(quant, prop, atoms, per_atom):
                     raise ValueError("/atom only possible in config_properties")
                 data = at.arrays
 
-            # grab data, make a copy so normalization doesn't affect original
-            ref_quant = np.asarray(data.get(ref_property_prefix + prop_use)).copy()
-            calc_quant = np.asarray(data.get(calc_property_prefix + prop_use)).copy()
+            # grab data
+            ref_quant = data.get(ref_property_prefix + prop_use)
+            calc_quant = data.get(calc_property_prefix + prop_use)
             if ref_quant is None or calc_quant is None:
                 # warn if data is missing by reporting summary at the very end
                 if prop not in missed_prop_counter:
@@ -165,6 +165,10 @@ def _reshape_normalize(quant, prop, atoms, per_atom):
 
                 continue
 
+            # make a copy so normalization doesn't affect original
+            ref_quant = np.asarray(ref_quant).copy()
+            calc_quant = np.asarray(calc_quant).copy()
+
             if virial_from_stress:
                 # ref quant was actually stress, automatically convert
                 ref_quant *= -at.get_volume()