diff --git a/src/calibre/ebooks/metadata/sources/identify.py b/src/calibre/ebooks/metadata/sources/identify.py
index 81f3e8247f..849dbd1555 100644
--- a/src/calibre/ebooks/metadata/sources/identify.py
+++ b/src/calibre/ebooks/metadata/sources/identify.py
@@ -87,7 +87,7 @@ class ISBNMerge(object):
                     isbns = frozenset([isbn])
                 if isbns in self.pools:
                     # xISBN had a brain fart
-                    pool = self.pools[isbn]
+                    pool = self.pools[isbns]
                 else:
                     self.pools[isbns] = pool = (min_year, [])