diff --git a/libraries/liblmdb/mdb.c b/libraries/liblmdb/mdb.c
index a6b5375c00..9c1dd91183 100644
--- a/libraries/liblmdb/mdb.c
+++ b/libraries/liblmdb/mdb.c
@@ -137,6 +137,11 @@ typedef SSIZE_T	ssize_t;
 # define MDB_FDATASYNC		fsync
 #endif
 
+/* NetBSD does not define union semun in sys/sem.h */
+#if defined(__NetBSD__) && !defined(_SEM_SEMUN_UNDEFINED)
+# define _SEM_SEMUN_UNDEFINED  1
+#endif
+
 #ifndef _WIN32
 #include <pthread.h>
 #include <signal.h>