diff --git a/src/qp/factory.py b/src/qp/factory.py
index f1fbf02..d00240c 100644
--- a/src/qp/factory.py
+++ b/src/qp/factory.py
@@ -363,7 +363,7 @@ def write_dict(filename, ensemble_dict, **kwargs):
         for key, val in ensemble_dict.items():
             # check that val is a qp.Ensemble
             if not isinstance(val, Ensemble):
-                raise ValueError("All values in ensemble_dict must be qp.Ensemble")
+                raise ValueError("All values in ensemble_dict must be qp.Ensemble") # pragma: no cover
 
             output_tables[key] = val.build_tables()
         io.writeDictsToHdf5(output_tables, filename, **kwargs)