diff --git a/src/types/function.rs b/src/types/function.rs
index 4772e5aaf04..fb31b087f29 100644
--- a/src/types/function.rs
+++ b/src/types/function.rs
@@ -21,14 +21,13 @@ unsafe extern "C" fn run_closure<F, R>(
     kwargs: *mut ffi::PyObject,
 ) -> *mut ffi::PyObject
 where
-    F: FnMut(&types::PyTuple, Option<&types::PyDict>) -> R + Send + 'static,
+    F: Fn(&types::PyTuple, Option<&types::PyDict>) -> R + Send + 'static,
     R: crate::callback::IntoPyCallbackOutput<*mut ffi::PyObject>,
 {
     crate::callback_body!(py, {
-        let boxed_fn: &mut F = &mut *(ffi::PyCapsule_GetPointer(
-            capsule_ptr,
-            CLOSURE_CAPSULE_NAME.as_ptr() as *const _,
-        ) as *mut F);
+        let boxed_fn: &F =
+            &*(ffi::PyCapsule_GetPointer(capsule_ptr, CLOSURE_CAPSULE_NAME.as_ptr() as *const _)
+                as *mut F);
         let args = py.from_borrowed_ptr::<types::PyTuple>(args);
         let kwargs = py.from_borrowed_ptr_or_opt::<types::PyDict>(kwargs);
         boxed_fn(args, kwargs)
@@ -37,7 +36,7 @@ where
 
 unsafe extern "C" fn drop_closure<F, R>(capsule_ptr: *mut ffi::PyObject)
 where
-    F: FnMut(&types::PyTuple, Option<&types::PyDict>) -> R + Send + 'static,
+    F: Fn(&types::PyTuple, Option<&types::PyDict>) -> R + Send + 'static,
     R: crate::callback::IntoPyCallbackOutput<*mut ffi::PyObject>,
 {
     let result = std::panic::catch_unwind(|| {
@@ -102,7 +101,7 @@ impl PyCFunction {
     /// ```
     pub fn new_closure<F, R>(f: F, py: Python) -> PyResult<&PyCFunction>
     where
-        F: FnMut(&types::PyTuple, Option<&types::PyDict>) -> R + Send + 'static,
+        F: Fn(&types::PyTuple, Option<&types::PyDict>) -> R + Send + 'static,
         R: crate::callback::IntoPyCallbackOutput<*mut ffi::PyObject>,
     {
         let function_ptr = Box::into_raw(Box::new(f));
diff --git a/tests/test_pyfunction.rs b/tests/test_pyfunction.rs
index 2e7437eb61b..2fe675b4c60 100644
--- a/tests/test_pyfunction.rs
+++ b/tests/test_pyfunction.rs
@@ -254,9 +254,10 @@ fn test_closure_counter() {
     let gil = Python::acquire_gil();
     let py = gil.python();
 
-    let mut counter = Box::new(0);
+    let counter = std::cell::RefCell::new(0);
     let counter_fn =
         move |_args: &types::PyTuple, _kwargs: Option<&types::PyDict>| -> PyResult<i32> {
+            let mut counter = counter.borrow_mut();
             *counter += 1;
             Ok(*counter)
         };