diff --git a/mariadb/connectionpool.py b/mariadb/connectionpool.py
index 882ff69..b17c984 100644
--- a/mariadb/connectionpool.py
+++ b/mariadb/connectionpool.py
@@ -218,7 +218,8 @@ class ConnectionPool(object):
 
                 conn._used += 1
                 self._connections_used.append(conn)
-                del self._connections_free[i]
+                x = self._connections_free.index(conn)
+                del self._connections_free[x]
                 return conn
 
         return None