diff --git a/modules/mappers/mod_so.c b/modules/mappers/mod_so.c
index cd5c4724ff..9d122c41c0 100644
--- a/modules/mappers/mod_so.c
+++ b/modules/mappers/mod_so.c
@@ -129,6 +129,7 @@
 #include "httpd.h"
 #include "http_config.h"
 #include "http_log.h"
+#include "ap_config.h"
 
 module MODULE_VAR_EXPORT so_module;