diff --git a/gravity/process_manager/__init__.py b/gravity/process_manager/__init__.py
index <HASH>..<HASH> 100644
--- a/gravity/process_manager/__init__.py
+++ b/gravity/process_manager/__init__.py
@@ -106,9 +106,9 @@ class BaseProcessManager(object, metaclass=ABCMeta):
         registered_instance_names = self.config_manager.get_registered_instances()
         unknown_instance_names = []
         if instance_names:
-            for i, n in enumerate(instance_names):
+            for n in instance_names:
                 if n not in registered_instance_names:
-                    unknown_instance_names.append(instance_names.pop(i))
+                    unknown_instance_names.append(n)
         elif registered_instance_names:
             instance_names = registered_instance_names
         else:
Don't pop from passed in instance_names

We may be passing in tuples. Not sure which one is the bug, but not
manipulating passed in values wins by default for me.