diff --git a/src/testdir/test_assert.vim b/src/testdir/test_assert.vim
index 5037d87..2147e3d 100644
--- a/src/testdir/test_assert.vim
+++ b/src/testdir/test_assert.vim
@@ -78,11 +78,18 @@
   call writefile(['1234X89'], 'Xone')
   call writefile(['1234Y89'], 'Xtwo')
   call assert_equal(1, assert_equalfile('Xone', 'Xtwo'))
-  call assert_match("difference at byte 4", v:errors[0])
+  call assert_match('difference at byte 4, line 1 after "1234X" vs "1234Y"', v:errors[0])
+  call remove(v:errors, 0)
+
+  call writefile([repeat('x', 234) .. 'X'], 'Xone')
+  call writefile([repeat('x', 234) .. 'Y'], 'Xtwo')
+  call assert_equal(1, assert_equalfile('Xone', 'Xtwo'))
+  let xes = repeat('x', 134)
+  call assert_match('difference at byte 234, line 1 after "' .. xes .. 'X" vs "' .. xes .. 'Y"', v:errors[0])
   call remove(v:errors, 0)
 
   call assert_equal(1, assert_equalfile('Xone', 'Xtwo', 'a message'))
-  call assert_match("a message: difference at byte 4", v:errors[0])
+  call assert_match("a message: difference at byte 234, line 1 after", v:errors[0])
   call remove(v:errors, 0)
 
   call delete('Xone')