diff --git a/psutil/_psutil_osx.c b/psutil/_psutil_osx.c
index 2cea8053e..7a0f73b01 100644
--- a/psutil/_psutil_osx.c
+++ b/psutil/_psutil_osx.c
@@ -1226,7 +1226,7 @@ psutil_proc_open_files(PyObject *self, PyObject *args) {
         free(fds_pointer);
     if (errno != 0)
         return PyErr_SetFromErrno(PyExc_OSError);
-    else if (! psutil_pid_exists(pid))
+    else if (psutil_pid_exists(pid) == 0)
         return NoSuchProcess();
     else
         return NULL;  // exception has already been set earlier