diff --git a/sphinx/environment.py b/sphinx/environment.py
index 214dd8223..fad5f2b36 100644
--- a/sphinx/environment.py
+++ b/sphinx/environment.py
@@ -1701,7 +1701,7 @@ class BuildEnvironment:
         rewrite_needed = []
 
         assigned = set()
-        old_fignumbers = self.toc_fignumbers
+        old_fignumbers = getattr(self, 'toc_fignumbers', {})  # compatible with old envs
         self.toc_fignumbers = {}
         fignum_counter = {}