diff --git a/plugins/calibre.koplugin/search.lua b/plugins/calibre.koplugin/search.lua
index ff9805814..076045009 100644
--- a/plugins/calibre.koplugin/search.lua
+++ b/plugins/calibre.koplugin/search.lua
@@ -271,7 +271,7 @@ function CalibreSearch:bookCatalog(t, option)
         local entry = {}
         entry.info = getBookInfo(book)
         entry.path = book.rootpath .. "/" .. book.lpath
-        if series then
+        if series and book.series_index then
             local major, minor = string.format("%05.2f", book.series_index):match("([^.]+).([^.]+)")
             if minor ~= "00" then
                 subseries = true