diff --git a/cache.h b/cache.h
index 89aaf0022d..b1adbe7b6e 100644
--- a/cache.h
+++ b/cache.h
@@ -24,6 +24,9 @@
 #define DTYPE(de)	DT_UNKNOWN
 #endif
 
+/* unknown mode (impossible combination S_IFIFO|S_IFCHR) */
+#define S_IFINVALID     0030000
+
 /*
  * A "directory link" is a link to another git directory.
  *