diff --git a/parfor/__init__.py b/parfor/__init__.py
index 3a5a059..0bdaae1 100644
--- a/parfor/__init__.py
+++ b/parfor/__init__.py
@@ -375,7 +375,8 @@ class PoolSingleton:
     def __new__(cls, n_processes: int = None, *args: Any, **kwargs: Any) -> PoolSingleton:
         # restart if any workers have shut down or if we want to have a different number of processes
         if cls.instance is not None:
-            if cls.instance.n_workers.value < cls.instance.n_processes or cls.instance.n_processes != n_processes:
+            if (cls.instance.n_workers.value < cls.instance.n_processes or
+                    cls.instance.n_processes != (n_processes or cpu_count)):
                 cls.instance.close()
         if cls.instance is None or not cls.instance.is_alive:
             new = super().__new__(cls)
diff --git a/pyproject.toml b/pyproject.toml
index bb40e8e..7e3c05a 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "parfor"
-version = "2024.5.0"
+version = "2024.5.1"
 description = "A package to mimic the use of parfor as done in Matlab."
 authors = ["Wim Pomp <wimpomp@gmail.com>"]
 license = "GPLv3"