1 files changed, 2 insertions, 0 deletions

diff --git a/src/fileio.c b/src/fileio.c
index 5e6f048b9c2..ff79504430c 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -174,6 +174,8 @@ A file name is handled if one of the regular expressions in\n\
 {
   /* This function must not munge the match data.  */
 
+  CHECK_STRING (filename, 0);
+
   Lisp_Object chain;
   for (chain = Vfile_name_handler_alist; XTYPE (chain) == Lisp_Cons;
        chain = XCONS (chain)->cdr)