diff --git a/openhtf/core/phase_descriptor.py b/openhtf/core/phase_descriptor.py
index 1c558cc90..cbc1d9fac 100644
--- a/openhtf/core/phase_descriptor.py
+++ b/openhtf/core/phase_descriptor.py
@@ -239,8 +239,7 @@ def _apply_with_plugs(self, subplugs, error_on_unknown):
         # Check __dict__ to see if the attribute is explicitly defined in the
         # class, rather than being defined in a parent class.
         accept_substitute = ('auto_placeholder' in original_plug.cls.__dict__
-                             and original_plug.cls.auto_placeholder
-                             and issubclass(sub_class, original_plug.cls))
+                             and original_plug.cls.auto_placeholder)
 
       if not accept_substitute:
         raise openhtf.plugs.InvalidPlugError(