diff --git a/src/config/conf.c b/src/config/conf.c
index 92fd7d123..1e1ca2e29 100644
--- a/src/config/conf.c
+++ b/src/config/conf.c
@@ -1265,7 +1265,7 @@ write_config_file(char *prefix, char *name,
 	int prefixlen = strlen(prefix);
 	int prefix_has_slash = (prefixlen && dir_sep(prefix[prefixlen - 1]));
 	int name_has_slash = dir_sep(name[0]);
-	char *slash = name_has_slash || prefix_has_slash ? "" : STRING_DIR_SEP;
+	const char *slash = name_has_slash || prefix_has_slash ? "" : STRING_DIR_SEP;
 
 	if (!cfg_str) return -1;