diff --git a/pygsti/protocols/gst.py b/pygsti/protocols/gst.py
index 79f0b142f..c013fe641 100644
--- a/pygsti/protocols/gst.py
+++ b/pygsti/protocols/gst.py
@@ -1864,7 +1864,7 @@ def run(self, data, memlimit=None, comm=None, checkpoint=None, checkpoint_path=N
 
                     #Try to interpret `mode` as a parameterization
                     parameterization = mode  # for now, 1-1 correspondence
-                    initial_model = target_model
+                    initial_model = target_model.copy()
 
                     try:
                         initial_model.set_all_parameterizations(parameterization)