diff --git a/libraries/liblmdb/lmdb.h b/libraries/liblmdb/lmdb.h
index 2374ef7..a6c53a9 100644
--- a/libraries/liblmdb/lmdb.h
+++ b/libraries/liblmdb/lmdb.h
@@ -727,6 +727,7 @@ void mdb_env_close(MDB_env *env);
 	 * This may be used to set some flags in addition to those from
 	 * #mdb_env_open(), or to unset these flags.  If several threads
 	 * change the flags at the same time, the result is undefined.
+	 * Most flags cannot be changed after #mdb_env_open().
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[in] flags The flags to change, bitwise OR'ed together
 	 * @param[in] onoff A non-zero value sets the flags, zero clears them.
diff --git a/libraries/liblmdb/mdb.c b/libraries/liblmdb/mdb.c
index 4a21242..3430ec1 100644
--- a/libraries/liblmdb/mdb.c
+++ b/libraries/liblmdb/mdb.c
@@ -8715,7 +8715,7 @@ mdb_env_copy(MDB_env *env, const char *path)
 int ESECT
 mdb_env_set_flags(MDB_env *env, unsigned int flag, int onoff)
 {
-	if ((flag & CHANGEABLE) != flag)
+	if (flag & (env->me_map ? ~CHANGEABLE : ~(CHANGEABLE|CHANGELESS)))
 		return EINVAL;
 	if (onoff)
 		env->me_flags |= flag;