diff -ur a/a/lib.c b/b/lib.c
--- a/a/lib.c
+++ b/b/lib.c
@@ -705,7 +705,7 @@ void mutt_sanitize_filename (char *f, short slash)

   for (; *f; f++)
   {
-    if ((slash && *f == '/'))
+    if (slash && *f == '/') || (*f == ' ')
       *f = '_';
   }
 }