diff --git a/setup.py b/setup.py
index 8095e3b7f..99e501d9f 100644
--- a/setup.py
+++ b/setup.py
@@ -912,7 +912,8 @@ class PySpecCommand(Command):
 
         if not self.dry_run:
             with open(os.path.join(self.out_dir, '__init__.py'), 'w') as out:
-                out.write("")
+                # `mainnet` is the default spec.
+                out.write("from . import mainnet as spec  # noqa:F401\n")
 
 
 class BuildPyCommand(build_py):