diff --git a/CHANGES b/CHANGES
index b544ad895..d75cfad9f 100644
--- a/CHANGES
+++ b/CHANGES
@@ -20,6 +20,7 @@ Bugs fixed
   as a value
 * #7156: autodoc: separator for keyword only arguments is not shown
 * #7146: autodoc: IndexError is raised on suppressed type_comment found
+* #7151: crashed when extension assigns a value to ``env.indexentries``
 
 Testing
 --------
diff --git a/sphinx/environment/__init__.py b/sphinx/environment/__init__.py
index 4ccdce7f1..f1ae6024d 100644
--- a/sphinx/environment/__init__.py
+++ b/sphinx/environment/__init__.py
@@ -755,6 +755,14 @@ class BuildEnvironment:
         domain = cast(IndexDomain, self.get_domain('index'))
         return domain.entries
 
+    @indexentries.setter
+    def indexentries(self, entries: Dict[str, List[Tuple[str, str, str, str, str]]]) -> None:
+        warnings.warn('env.indexentries() is deprecated. Please use IndexDomain instead.',
+                      RemovedInSphinx40Warning, stacklevel=2)
+        from sphinx.domains.index import IndexDomain
+        domain = cast(IndexDomain, self.get_domain('index'))
+        domain.data['entries'] = entries
+
 
 from sphinx.errors import NoUri  # NOQA