--- a/util-linux/mount.c
+++ b/util-linux/mount.c
@@ -42,6 +42,10 @@
 #define MS_DIRSYNC      128     // Directory modifications are synchronous
 #endif
 
+#ifndef MS_UNION
+#define MS_UNION	0x100   /* 256: Mount on top of a union */
+#endif
+
 
 #if defined(__dietlibc__)
 // 16.12.2006, Sampo Kellomaki (sampo@iki.fi)
@@ -152,6 +156,7 @@ static const int32_t mount_options[] = {
 		/* "rslave"      */ MS_SLAVE|MS_RECURSIVE,
 		/* "rprivate"    */ MS_SLAVE|MS_RECURSIVE,
 		/* "runbindable" */ MS_UNBINDABLE|MS_RECURSIVE,
+		/* "union"       */ MS_UNION,
 	)
 
 	// Always understood.
@@ -206,6 +211,7 @@ static const char mount_option_str[] =
 		"rslave\0"
 		"rprivate\0"
 		"runbindable\0"
+		"union" "\0"
 	)
 
 	// Always understood.