diff --git a/sources.c b/sources.c
index 0fa9eeb..9865908 100644
--- a/sources.c
+++ b/sources.c
@@ -483,7 +483,7 @@ SRC_ResetReachability(SRC_Instance inst)
 /* ================================================== */
 
 static void
-log_selection_message(char *format, char *arg)
+log_selection_message(const char *format, const char *arg)
 {
   if (REF_GetMode() != REF_ModeNormal)
     return;