diff --git a/CHANGES b/CHANGES
index a3dca85e9..48d95d852 100644
--- a/CHANGES
+++ b/CHANGES
@@ -33,6 +33,7 @@ Bugs fixed
 * #4871: html search: Upper characters problem in German
 * #4717: latex: Compilation for German docs failed with LuaLaTeX and XeLaTeX
 * #4459: duplicated labels detector does not work well in parallel build
+* #4878: Crashed with extension which returns invalid metadata
 
 Testing
 --------
diff --git a/sphinx/registry.py b/sphinx/registry.py
index cc012b3f7..cdae77224 100644
--- a/sphinx/registry.py
+++ b/sphinx/registry.py
@@ -340,6 +340,7 @@ class SphinxComponentRegistry(object):
             logger.warning(__('extension %r returned an unsupported object from '
                               'its setup() function; it should return None or a '
                               'metadata dictionary'), extname)
+            metadata = {}
 
         app.extensions[extname] = Extension(extname, mod, **metadata)
         app._setting_up_extension.pop()