diff --git a/src/mygetopt.cpp b/src/mygetopt.cpp
index da1dba38..a685f6d8 100644
--- a/src/mygetopt.cpp
+++ b/src/mygetopt.cpp
@@ -530,7 +530,7 @@ static int _getopt_internal (int argc, char **argv, const char *optstring,
 
   {
     char c = *nextchar++;
-    char *temp = my_index (optstring, c);
+    const char *temp = my_index (optstring, c);
 
     /* Increment `optind' when we start to process its last character.  */
     if (*nextchar == '\0')