diff --git a/libowlps/libowlps.c b/libowlps/libowlps.c
index 02c0835..861cfbc 100644
--- a/libowlps/libowlps.c
+++ b/libowlps/libowlps.c
@@ -413,6 +413,8 @@ float owl_swap_float(const float f)
 
 /**
  * Opens a UDP transmission socket and returns its descriptor.
+ * Uppon error, a message is displayed and a negative error code is
+ * returned.
  *
  * @param[in] server_address The server's IP address.
  * @param[in] server_port The listening port on the server.
@@ -585,15 +587,16 @@ void owl_sigterm_handler(const int num)
 /**
  * Closes the file descriptor `fd`.
  * `fd` must be passed as an int pointer (`int*`). If `fd` is `NULL`,
- * nothing will be done. Uppon error, a message is displayed on the
- * standard error.
+ * or if the pointed value is negative, nothing will be done.
+ * Uppon error, a message is displayed on the standard error.
  */
 void owl_close_fd(void *const fd)
 {
-  if (fd == NULL)
+  const int *const file_desc = fd;
+
+  if (file_desc == NULL || *file_desc < 0)
     return ;
 
-  const int *const file_desc = fd ;
   if (close(*file_desc) != 0)
     perror("Error closing file descriptor") ;
 #ifndef NDEBUG