diff --git a/src/fileio_type.h b/src/fileio_type.h
--- a/src/fileio_type.h
+++ b/src/fileio_type.h
@@ -20,6 +20,7 @@ enum Subdirectory {
 	DATA_DIR,      ///< Subdirectory for all data (GRFs, sample.cat, intro game)
 	LANG_DIR,      ///< Subdirectory for all translation files
 	AI_DIR,        ///< Subdirectory for all AI files
+	AI_LIBRARY_DIR,///< Subdirectory for all AI libraries
 	NUM_SUBDIRS,   ///< Number of subdirectories
 	NO_DIRECTORY,  ///< A path without any base directory
 };