diff --git a/src/ocrmypdf/helpers.py b/src/ocrmypdf/helpers.py
index 1671ea7a..745549a8 100644
--- a/src/ocrmypdf/helpers.py
+++ b/src/ocrmypdf/helpers.py
@@ -71,7 +71,7 @@ def is_iterable_notstr(thing):
 
 def page_number(input_file):
     "Get one-based page number implied by filename (000002.pdf -> 2)"
-    return int(os.path.basename(fspath(input_file)[0:6]))
+    return int(os.path.basename(fspath(input_file))[0:6])
 
 
 def available_cpu_count():