diff --git a/src/sage_docbuild/builders.py b/src/sage_docbuild/builders.py
index b6b723976cd..9cc99254e64 100644
--- a/src/sage_docbuild/builders.py
+++ b/src/sage_docbuild/builders.py
@@ -1011,6 +1011,10 @@ def get_new_and_updated_modules(self):
                 raise
 
             module_filename = sys.modules[module_name].__file__
+            if module_filename is None:
+                # Namespace package
+                old_modules.append(module_name)
+                continue
             if (module_filename.endswith('.pyc') or module_filename.endswith('.pyo')):
                 source_filename = module_filename[:-1]
                 if (os.path.exists(source_filename)):