diff --git a/src/calibre/gui2/tweak_book/diff/view.py b/src/calibre/gui2/tweak_book/diff/view.py
index fa7f736c26..d1112dd260 100644
--- a/src/calibre/gui2/tweak_book/diff/view.py
+++ b/src/calibre/gui2/tweak_book/diff/view.py
@@ -631,9 +631,11 @@ class DiffSplit(QSplitter):  # {{{
                 v.images[top] = (img, w, lines)
                 def mapnum(x):
                     return x if x <= top else x + delta
-                lnm = v.line_number_map
-                for x, val in tuple(lnm.iteritems()):
+                lnm = LineNumberMap()
+                lnm.max_width = v.line_number_map.max_width
+                for x, val in v.line_number_map.iteritems():
                     dict.__setitem__(lnm, mapnum(x), val)
+                v.line_number_map = lnm
                 v.changes = [(mapnum(t), mapnum(b), kind) for t, b, kind in v.changes]
                 v.headers = [(mapnum(x), name) for x, name in v.headers]
                 v.images = OrderedDict((mapnum(x), v) for x, v in v.images.iteritems())