diff --git a/libraries/libmdb/mdb.c b/libraries/libmdb/mdb.c
index b73541b..1a539ba 100644
--- a/libraries/libmdb/mdb.c
+++ b/libraries/libmdb/mdb.c
@@ -436,7 +436,7 @@ typedef struct MDB_txbody {
 		 *	This always records the maximum count, it is not decremented
 		 *	when readers release their slots.
 		 */
-	uint32_t	mtb_numreaders;
+	unsigned	mtb_numreaders;
 		/**	The ID of the most recent meta page in the database.
 		 *	This is recorded here only for convenience; the value can always
 		 *	be determined by reading the main database meta pages.
@@ -1819,16 +1819,16 @@ mdb_env_set_maxdbs(MDB_env *env, int dbs)
 }
 
 int
-mdb_env_set_maxreaders(MDB_env *env, int readers)
+mdb_env_set_maxreaders(MDB_env *env, unsigned int readers)
 {
-	if (env->me_map)
+	if (env->me_map || readers < 1)
 		return EINVAL;
 	env->me_maxreaders = readers;
 	return MDB_SUCCESS;
 }
 
 int
-mdb_env_get_maxreaders(MDB_env *env, int *readers)
+mdb_env_get_maxreaders(MDB_env *env, unsigned int *readers)
 {
 	if (!env || !readers)
 		return EINVAL;
diff --git a/libraries/libmdb/mdb.h b/libraries/libmdb/mdb.h
index c0d4edc..f718c35 100644
--- a/libraries/libmdb/mdb.h
+++ b/libraries/libmdb/mdb.h
@@ -414,7 +414,7 @@ int  mdb_env_set_mapsize(MDB_env *env, size_t size);
 	 *	<li>EINVAL - an invalid parameter was specified, or the environment is already open.
 	 * </ul>
 	 */
-int  mdb_env_set_maxreaders(MDB_env *env, int readers);
+int  mdb_env_set_maxreaders(MDB_env *env, unsigned int readers);
 
 	/** Get the maximum number of threads for the environment.
 	 * @param[in] env An environment handle returned by #mdb_env_create()
@@ -425,7 +425,7 @@ int  mdb_env_set_maxreaders(MDB_env *env, int readers);
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 * </ul>
 	 */
-int  mdb_env_get_maxreaders(MDB_env *env, int *readers);
+int  mdb_env_get_maxreaders(MDB_env *env, unsigned int *readers);
 
 	/** Set the maximum number of databases for the environment.
 	 * This function is only needed if multiple databases will be used in the