diff --git a/libraries/liblmdb/mdb.c b/libraries/liblmdb/mdb.c
index 67defdb..cae20de 100644
--- a/libraries/liblmdb/mdb.c
+++ b/libraries/liblmdb/mdb.c
@@ -342,19 +342,25 @@ static txnid_t mdb_debug_start;
 	/**	The version number for a database's file format. */
 #define MDB_VERSION	 1
 
-	/**	The maximum size of a key in the database.
+	/**	@brief The maximum size of a key in the database.
+	 *
 	 *	While data items have essentially unbounded size, we require that
 	 *	keys all fit onto a regular page. This limit could be raised a bit
 	 *	further if needed; to something just under #MDB_PAGESIZE / #MDB_MINKEYS.
+	 *
+	 *	Note that data items in an #MDB_DUPSORT database are actually keys
+	 *	of a subDB, so they're also limited to this size.
 	 */
-#define MAXKEYSIZE	 511
+#ifndef MDB_MAXKEYSIZE
+#define MDB_MAXKEYSIZE	 511
+#endif
 
 #if MDB_DEBUG
 	/**	A key buffer.
 	 *	@ingroup debug
 	 *	This is used for printing a hex dump of a key's contents.
 	 */
-#define DKBUF	char kbuf[(MAXKEYSIZE*2+1)]
+#define DKBUF	char kbuf[(MDB_MAXKEYSIZE*2+1)]
 	/**	Display a key in hex.
 	 *	@ingroup debug
 	 *	Invoke a function to display a key in hex.
@@ -1081,8 +1087,8 @@ mdb_dkey(MDB_val *key, char *buf)
 	char *ptr = buf;
 	unsigned char *c = key->mv_data;
 	unsigned int i;
-	if (key->mv_size > MAXKEYSIZE)
-		return "MAXKEYSIZE";
+	if (key->mv_size > MDB_MAXKEYSIZE)
+		return "MDB_MAXKEYSIZE";
 	/* may want to make this a dynamic check: if the key is mostly
 	 * printable characters, print it as-is instead of converting to hex.
 	 */
@@ -2176,7 +2182,7 @@ free2:
 		MDB_val key, data;
 
 		/* make sure last page of freeDB is touched and on freelist */
-		key.mv_size = MAXKEYSIZE+1;
+		key.mv_size = MDB_MAXKEYSIZE+1;
 		key.mv_data = NULL;
 		rc = mdb_page_search(&mc, &key, MDB_PS_MODIFY);
 		if (rc && rc != MDB_NOTFOUND)
@@ -3954,7 +3960,7 @@ mdb_page_search_root(MDB_cursor *mc, MDB_val *key, int modify)
 
 		if (key == NULL)	/* Initialize cursor to first page. */
 			i = 0;
-		else if (key->mv_size > MAXKEYSIZE && key->mv_data == NULL) {
+		else if (key->mv_size > MDB_MAXKEYSIZE && key->mv_data == NULL) {
 							/* cursor to last page */
 			i = NUMKEYS(mp)-1;
 		} else {
@@ -4130,7 +4136,7 @@ mdb_get(MDB_txn *txn, MDB_dbi dbi,
 	if (txn == NULL || !dbi || dbi >= txn->mt_numdbs)
 		return EINVAL;
 
-	if (key->mv_size == 0 || key->mv_size > MAXKEYSIZE) {
+	if (key->mv_size == 0 || key->mv_size > MDB_MAXKEYSIZE) {
 		return EINVAL;
 	}
 
@@ -4572,7 +4578,7 @@ mdb_cursor_last(MDB_cursor *mc, MDB_val *key, MDB_val *data)
 	if (!(mc->mc_flags & C_INITIALIZED) || mc->mc_top) {
 		MDB_val	lkey;
 
-		lkey.mv_size = MAXKEYSIZE+1;
+		lkey.mv_size = MDB_MAXKEYSIZE+1;
 		lkey.mv_data = NULL;
 		rc = mdb_page_search(mc, &lkey, 0);
 		if (rc != MDB_SUCCESS)
@@ -4656,7 +4662,7 @@ mdb_cursor_get(MDB_cursor *mc, MDB_val *key, MDB_val *data,
 	case MDB_SET:
 	case MDB_SET_KEY:
 	case MDB_SET_RANGE:
-		if (key == NULL || key->mv_size == 0 || key->mv_size > MAXKEYSIZE) {
+		if (key == NULL || key->mv_size == 0 || key->mv_size > MDB_MAXKEYSIZE) {
 			rc = EINVAL;
 		} else if (op == MDB_SET_RANGE)
 			rc = mdb_cursor_set(mc, key, data, op, NULL);
@@ -4793,13 +4799,16 @@ mdb_cursor_put(MDB_cursor *mc, MDB_val *key, MDB_val *data,
 	size_t nsize;
 	int rc, rc2;
 	MDB_pagebuf pbuf;
-	char dbuf[MAXKEYSIZE+1];
+	char dbuf[MDB_MAXKEYSIZE+1];
 	unsigned int nflags;
 	DKBUF;
 
 	if (F_ISSET(mc->mc_txn->mt_flags, MDB_TXN_RDONLY))
 		return EACCES;
 
+	if (key->mv_size == 0 || key->mv_size > MDB_MAXKEYSIZE)
+		return EINVAL;
+
 	DPRINTF("==> put db %u key [%s], size %zu, data size %zu",
 		mc->mc_dbi, DKEY(key), key ? key->mv_size:0, data->mv_size);
 
@@ -5769,7 +5778,7 @@ mdb_update_key(MDB_page *mp, indx_t indx, MDB_val *key)
 #if MDB_DEBUG
 	{
 		MDB_val	k2;
-		char kbuf2[(MAXKEYSIZE*2+1)];
+		char kbuf2[(MDB_MAXKEYSIZE*2+1)];
 		k2.mv_data = NODEKEY(node);
 		k2.mv_size = node->mn_ksize;
 		DPRINTF("update key %u (ofs %u) [%s] to [%s] on page %zu",
@@ -6321,7 +6330,7 @@ mdb_del(MDB_txn *txn, MDB_dbi dbi,
 		return EACCES;
 	}
 
-	if (key->mv_size == 0 || key->mv_size > MAXKEYSIZE) {
+	if (key->mv_size == 0 || key->mv_size > MDB_MAXKEYSIZE) {
 		return EINVAL;
 	}
 
@@ -6760,7 +6769,7 @@ mdb_put(MDB_txn *txn, MDB_dbi dbi,
 		return EACCES;
 	}
 
-	if (key->mv_size == 0 || key->mv_size > MAXKEYSIZE) {
+	if (key->mv_size == 0 || key->mv_size > MDB_MAXKEYSIZE) {
 		return EINVAL;
 	}