diff --git a/templates/books_for_series.html b/templates/books_for_series.html
index 4bed8ed..9c6122e 100644
--- a/templates/books_for_series.html
+++ b/templates/books_for_series.html
@@ -20,7 +20,7 @@
 					<a href="/book/{{book.id}}">{{book.title}}</a>
 				{% endif %}
 			</td>
-			<td data-sort="{{series.id}}.{{"%02d".format( SeriesBookNum( series.id, book.id ))}}">
+			<td data-sort="{{series.id}}.{{"{:02d}".format( SeriesBookNum( series.id, book.id ))}}">
 			{% for s in book.series %}
 				{{ SeriesBookNum( s.id, book.id ) }} of <a href="/series/{{s.id}}">{{s.title}}</a><br>
 			{% endfor %}