diff --git a/src/calibre/ebooks/metadata/book/formatter.py b/src/calibre/ebooks/metadata/book/formatter.py
index 7adbe81016..5d9e2ebe6c 100644
--- a/src/calibre/ebooks/metadata/book/formatter.py
+++ b/src/calibre/ebooks/metadata/book/formatter.py
@@ -33,8 +33,7 @@ class SafeFormat(TemplateFormatter):
             b = self.book.get_user_metadata(key, False)
         except:
             b = None
-        if b and ((b['datatype'] == 'int' and self.book.get(key, 0) == 0) or
-                  (b['datatype'] == 'float' and self.book.get(key, 0.0) == 0.0)):
+        if b and b['datatype'] in {'int', 'float'} and self.book.get(key, None) is None:
             v = ''
         else:
             v = self.book.format_field(key, series_with_index=False)[1]
diff --git a/src/calibre/library/field_metadata.py b/src/calibre/library/field_metadata.py
index a76d78da0b..b59e28a847 100644
--- a/src/calibre/library/field_metadata.py
+++ b/src/calibre/library/field_metadata.py
@@ -504,9 +504,12 @@ class FieldMetadata(dict):
         return [k for k in self._tb_cats.iterkeys() if self.is_ignorable_field(k)]
 
     def is_series_index(self, key):
-        m = self[key]
-        return (m['datatype'] == 'float' and key.endswith('_index') and
-                key[:-6] in self)
+        try:
+            m = self._tb_cats[key]
+            return (m['datatype'] == 'float' and key.endswith('_index') and
+                    key[:-6] in self._tb_cats)
+        except (KeyError, ValueError, TypeError, AttributeError):
+            return False
 
     def key_to_label(self, key):
         if 'label' not in self._tb_cats[key]: