diff --git a/mutmut/__main__.py b/mutmut/__main__.py
index 89bd0b8d..1b778e9c 100644
--- a/mutmut/__main__.py
+++ b/mutmut/__main__.py
@@ -797,7 +797,7 @@ def orig_function_and_class_names_from_key(mutant_name):
         class_name = r[r.index(CLASS_NAME_SEPARATOR) + 1: r.rindex(CLASS_NAME_SEPARATOR)]
         r = r[r.rindex(CLASS_NAME_SEPARATOR) + 1:]
     else:
-        assert r.startswith('x_')
+        assert r.startswith('x_'),r
         r = r[2:]
     return r, class_name
 
@@ -1544,7 +1544,9 @@ def load_thread():
                             if event.row_key.value == self.loading_id:
                                 diff_view.text = d
                         except Exception as e:
-                            diff_view.text = f'<{e}>'
+                            import traceback
+
+                            diff_view.text = traceback.format_exc()
 
                     t = Thread(target=load_thread)
                     t.start()