diff --git a/parfor/__init__.py b/parfor/__init__.py
index e4ee1cb..c60dae8 100644
--- a/parfor/__init__.py
+++ b/parfor/__init__.py
@@ -81,7 +81,10 @@ class SharedMemory(UserDict):
     def garbage_collect(self):
         """ clean up the cache """
         for item_id in set(self.cache) - set(self.data.keys()):
-            self.references[item_id] -= 1
+            if item_id in self.references:
+                self.references[item_id] -= 1
+            else:
+                self.references[item_id] = 0
             if self.trash_can is not None and item_id not in self.trash_can:
                 self.trash_can[item_id] = self.cache[item_id]
             del self.cache[item_id]
diff --git a/pyproject.toml b/pyproject.toml
index 0b6b3f5..d7d585d 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "parfor"
-version = "2023.11.0"
+version = "2023.11.1"
 description = "A package to mimic the use of parfor as done in Matlab."
 authors = ["Wim Pomp <wimpomp@gmail.com>"]
 license = "GPLv3"