diff --git a/glib/gconvert.c b/glib/gconvert.c
index 609a800a9..fd669aa81 100644
--- a/glib/gconvert.c
+++ b/glib/gconvert.c
@@ -1469,6 +1469,8 @@ g_filename_to_utf8 (const gchar *opsysstring,
 {
   const gchar *charset;
 
+  g_return_val_if_fail (opsysstring != NULL, NULL);
+
   if (get_filename_charset (&charset))
     return strdup_len (opsysstring, len, bytes_read, bytes_written, error);
   else
@@ -1494,6 +1496,8 @@ g_filename_to_utf8 (const gchar *opsysstring,
 {
   const gchar *charset;
 
+  g_return_val_if_fail (opsysstring != NULL, NULL);
+
   if (g_get_charset (&charset))
     return strdup_len (opsysstring, len, bytes_read, bytes_written, error);
   else