diff --git a/field.py b/field.py
index 4bb27ef..36aa591 100644
--- a/field.py
+++ b/field.py
@@ -1,6 +1,6 @@
 import numpy
 class Field3d:
-    def __init__(self,data,origin,spacing,period=(None,None,None)):
+    def __init__(self,data,origin,spacing,bounds,period):
         assert len(origin)==3,  "'origin' must be of length 3"
         assert len(spacing)==3, "'spacing' must be of length 3"
         assert len(period)==3,  "'period' must be of length 3"