diff --git a/src/pdbpp.py b/src/pdbpp.py
index 1ddaa115..e5a3bd03 100644
--- a/src/pdbpp.py
+++ b/src/pdbpp.py
@@ -274,7 +274,7 @@ def set_trace(
             return global_pdb
 
         obj = cls.__new__(cls)
-        if called_for_set_trace:
+        if cls is Pdb and called_for_set_trace:
             kwargs.setdefault("start_filename", called_for_set_trace.f_code.co_filename)
             kwargs.setdefault("start_lineno", called_for_set_trace.f_lineno)