diff --git a/src/minibuf.c b/src/minibuf.c
index 87cdcea4522..eeaad5583e6 100644
--- a/src/minibuf.c
+++ b/src/minibuf.c
@@ -1942,7 +1942,7 @@ Return nil if there is no valid completion, else t.  */)
 {
   Lisp_Object completion, tem, tem1;
   register int i, i_byte;
-  register unsigned char *completion_string;
+  register const unsigned char *completion_string;
   struct gcpro gcpro1, gcpro2;
   int prompt_end_charpos = XINT (Fminibuffer_prompt_end ());
 
@@ -2365,7 +2365,7 @@ If no minibuffer is active, return nil.  */)
 
 void
 temp_echo_area_glyphs (m)
-     char *m;
+     const char *m;
 {
   int osize = ZV;
   int osize_byte = ZV_BYTE;