diff --git a/libraries/liblmdb/lmdb.h b/libraries/liblmdb/lmdb.h
index 2ebd43f3da..2ece80580d 100644
--- a/libraries/liblmdb/lmdb.h
+++ b/libraries/liblmdb/lmdb.h
@@ -1333,7 +1333,7 @@ int  mdb_cursor_get(MDB_cursor *cursor, MDB_val *key, MDB_val *data,
 	 * <ul>
 	 *	<li>#MDB_CURRENT - overwrite the data of the key/data pair to which
 	 *		the cursor refers with the specified data item. The \b key
-	 *		parameter is ignored.
+	 *		parameter is ignored.  The \b data size must match the original.
 	 *	<li>#MDB_NODUPDATA - enter the new key/data pair only if it does not
 	 *		already appear in the database. This flag may only be specified
 	 *		if the database was opened with #MDB_DUPSORT. The function will
diff --git a/libraries/liblmdb/mdb.c b/libraries/liblmdb/mdb.c
index 7e2905891e..5a607a4c7e 100644
--- a/libraries/liblmdb/mdb.c
+++ b/libraries/liblmdb/mdb.c
@@ -2728,10 +2728,11 @@ mdb_freelist_save(MDB_txn *txn)
 			MDB_ID save;
 
 			mdb_tassert(txn, len >= 0 && id <= env->me_pglast);
-			key.mv_data = &id;
 			if (len > mop_len) {
 				len = mop_len;
 				data.mv_size = (len + 1) * sizeof(MDB_ID);
+				/* Drop MDB_CURRENT when changing the data size */
+				key.mv_data = &id;
 				flags = 0;
 			}
 			data.mv_data = mop -= len;
@@ -5761,7 +5762,7 @@ mdb_cursor_put(MDB_cursor *mc, MDB_val *key, MDB_val *data,
 	unsigned int nflags;
 	DKBUF;
 
-	if (mc == NULL || key == NULL)
+	if (mc == NULL)
 		return EINVAL;
 
 	env = mc->mc_txn->mt_env;
@@ -5782,8 +5783,16 @@ mdb_cursor_put(MDB_cursor *mc, MDB_val *key, MDB_val *data,
 	if (mc->mc_txn->mt_flags & (MDB_TXN_RDONLY|MDB_TXN_ERROR))
 		return (mc->mc_txn->mt_flags & MDB_TXN_RDONLY) ? EACCES : MDB_BAD_TXN;
 
-	if (flags != MDB_CURRENT && key->mv_size-1 >= ENV_MAXKEY(env))
-		return MDB_BAD_VALSIZE;
+	if (flags != MDB_CURRENT) {
+		if (key == NULL)
+			return EINVAL;
+		if (key->mv_size-1 >= ENV_MAXKEY(env))
+			return MDB_BAD_VALSIZE;
+	} else {
+		/* Ignore key except in sub-cursor, where key holds the data */
+		if (!(mc->mc_flags & C_SUB))
+			key = NULL;
+	}
 
 #if SIZE_MAX > MAXDATASIZE
 	if (data->mv_size > ((mc->mc_db->md_flags & MDB_DUPSORT) ? ENV_MAXKEY(env) : MAXDATASIZE))
@@ -6107,7 +6116,7 @@ current:
 			 */
 			if (F_ISSET(flags, MDB_RESERVE))
 				data->mv_data = olddata.mv_data;
-			else if (data->mv_size)
+			else if (!(mc->mc_flags & C_SUB))
 				memcpy(olddata.mv_data, data->mv_data, data->mv_size);
 			else
 				memcpy(NODEKEY(leaf), key->mv_data, key->mv_size);