diff --git a/libraries/liblmdb/midl.h b/libraries/liblmdb/midl.h
index aa45c2e..765708c 100644
--- a/libraries/liblmdb/midl.h
+++ b/libraries/liblmdb/midl.h
@@ -56,7 +56,9 @@ typedef MDB_ID *MDB_IDL;
 /* IDL sizes - likely should be even bigger
  *   limiting factors: sizeof(ID), thread stack size
  */
+#ifndef MDB_IDL_LOGN
 #define	MDB_IDL_LOGN	16	/* DB_SIZE is 2^16, UM_SIZE is 2^17 */
+#endif
 #define MDB_IDL_DB_SIZE		(1<<MDB_IDL_LOGN)
 #define MDB_IDL_UM_SIZE		(1<<(MDB_IDL_LOGN+1))