diff --git a/include/curl/curl.h b/include/curl/curl.h
index 5f85ffff6c5f9e7d12ca5abd1d728550a228773a..af0560f983f19caf7cafc46c71081adb67d064a9 100644
--- a/include/curl/curl.h
+++ b/include/curl/curl.h
@@ -1246,7 +1246,7 @@ typedef struct {
   int ares_num;
 
   /* This field was aded in CURLVERSION_THIRD */
-  char *libidn;
+  const char *libidn;
 } curl_version_info_data;
 
 #define CURL_VERSION_IPV6      (1<<0)  /* IPv6-enabled */