--- a/mercurial/templater.py	Sun Feb 14 12:48:14 2016 +0900
+++ b/mercurial/templater.py	Sun Feb 14 13:05:09 2016 +0900
@@ -398,7 +398,9 @@
         # i18n: "pad" is a keyword
         raise error.ParseError(_("pad() expects two to four arguments"))
 
-    width = int(args[1][1])
+    width = evalinteger(context, mapping, args[1],
+                        # i18n: "pad" is a keyword
+                        _("pad() expects an integer width"))
 
     text = stringify(args[0][0](context, mapping, args[0][1]))
 

--- a/tests/test-command-template.t	Sun Feb 14 12:48:14 2016 +0900
+++ b/tests/test-command-template.t	Sun Feb 14 13:05:09 2016 +0900
@@ -3244,6 +3244,14 @@
   $ hg log -r 0 -T '{pad(r"\{rev}", 10)} {author|user}\n'
   \{rev}     test
 
+Test width argument passed to pad function
+
+  $ hg log -r 0 -T '{pad(rev, "1{"0"}")} {author|user}\n'
+  0          test
+  $ hg log -r 0 -T '{pad(rev, "not an int")}\n'
+  hg: parse error: pad() expects an integer width
+  [255]
+
 Test ifcontains function
 
   $ hg log --template '{rev} {ifcontains(rev, "2 two 0", "is in the string", "is not")}\n'