diff --git a/pykat/utilities/optics/ABCD.py b/pykat/utilities/optics/ABCD.py
index 58ecc2e61951a564ec9f25a4d6f8a905d09f7203..1b9f348fba2aaa54b7689ab79bd97178f95d2b1a 100644
--- a/pykat/utilities/optics/ABCD.py
+++ b/pykat/utilities/optics/ABCD.py
@@ -1,7 +1,7 @@
 import numpy as np
 
 def apply(ABCD, q1, n1, n2):
-    return n2 * (ABCD[0,0] * q1/n1 + ABCD[0,1] / (ABCD[1,0] * q1/n1 + ABCD[1,1]))
+    return n2 * (ABCD[0,0] * q1/n1 + ABCD[0,1]) / (ABCD[1,0] * q1/n1 + ABCD[1,1])
 
 def mirror_trans(n1, n2, Rc):
     return np.matrix([[1,0],[(n2-n1)/Rc,1]])