diff --git a/cps/editbooks.py b/cps/editbooks.py
index 0cbdf751..cecaa9e2 100644
--- a/cps/editbooks.py
+++ b/cps/editbooks.py
@@ -439,6 +439,9 @@ def edit_book_series_index(series_index, book):
     # Add default series_index to book
     modif_date = False
     series_index = series_index or '1'
+    if not series_index.replace('.', '', 1).isdigit():
+        flash(_("%(seriesindex)s is not a valid number, skipping", seriesindex=series_index), category="warning")
+        return False
     if book.series_index != series_index:
         book.series_index = series_index
         modif_date = True
diff --git a/cps/jinjia.py b/cps/jinjia.py
index f37dfb49..de34cc86 100644
--- a/cps/jinjia.py
+++ b/cps/jinjia.py
@@ -122,9 +122,12 @@ def formatfloat(value, decimals=1):
 @jinjia.app_template_filter('formatseriesindex')
 def formatseriesindex_filter(series_index):
     if series_index:
-        if int(series_index) - series_index == 0:
-            return int(series_index)
-        else:
+        try:
+            if int(series_index) - series_index == 0:
+                return int(series_index)
+            else:
+                return series_index
+        except ValueError:
             return series_index
     return 0