diff --git a/src/calibre/ebooks/oeb/polish/container.py b/src/calibre/ebooks/oeb/polish/container.py
index 49dca6605c..9e41a1f1a5 100644
--- a/src/calibre/ebooks/oeb/polish/container.py
+++ b/src/calibre/ebooks/oeb/polish/container.py
@@ -344,7 +344,7 @@ class Container(ContainerBase):  # {{{
         item_id = 'id'
         while item_id in all_ids:
             c += 1
-            item_id = 'id' + f'{c}'
+            item_id = f'id{c}'
         manifest = self.opf_xpath('//opf:manifest')[0]
         href = self.name_to_href(name, self.opf_name)
         item = manifest.makeelement(OPF('item'),
diff --git a/src/calibre/ebooks/oeb/transforms/split.py b/src/calibre/ebooks/oeb/transforms/split.py
index 85381a000e..85b5ce1ec8 100644
--- a/src/calibre/ebooks/oeb/transforms/split.py
+++ b/src/calibre/ebooks/oeb/transforms/split.py
@@ -366,10 +366,10 @@ class FlowSplitter:
             elif size <= self.max_flow_size:
                 self.split_trees.append(t)
                 self.log.debug(
-                    f'\t\t\tCommitted sub-tree #{len(self.split_trees)} ({size / 1024.0} KB)')
+                    f'\t\t\tCommitted sub-tree #{len(self.split_trees)} ({size//1024} KB)')
             else:
                 self.log.debug(
-                        f'\t\t\tSplit tree still too large: {size / 1024.0} KB')
+                        f'\t\t\tSplit tree still too large: {size//1024} KB')
                 self.split_to_size(t)
 
     def find_split_point(self, root):