diff --git a/kwant/builder.py b/kwant/builder.py
index b43b5547d1d5212fdcd46b8d64fef46ef6d5382b..e33aba94c137b5b66b43cb95c92183531349881b 100644
--- a/kwant/builder.py
+++ b/kwant/builder.py
@@ -1071,7 +1071,7 @@ class Builder(object):
         the leads are numbered in the order in which they are attached.
         """
         if self.symmetry.num_directions:
-            raise ValueError("Can only attach leads to finite systems")
+            raise ValueError("Leads can only be attached to finite systems.")
 
         if add_cells < 0 or int(add_cells) != add_cells:
             raise ValueError('add_cells must be an integer >= 0.')