diff --git a/mpas_analysis/ocean/sose_transects.py b/mpas_analysis/ocean/sose_transects.py
index d9f2bee57..9850e4737 100644
--- a/mpas_analysis/ocean/sose_transects.py
+++ b/mpas_analysis/ocean/sose_transects.py
@@ -379,7 +379,7 @@ def combine_observations(self):
 
         # make a copy of the top set of data at z=0
         dsObs = xr.concat((dsObs.isel(z=0), dsObs), dim='z')
-        z = dsObs.z.values
+        z = dsObs.z.values.copy()
         z[0] = 0.
         dsObs['z'] = ('z', z)
         write_netcdf_with_fill(dsObs, combinedFileName)