diff --git a/changelog.d/6690.bugfix b/changelog.d/6690.bugfix
new file mode 100644
index 0000000000..30ce1dc9f7
--- /dev/null
+++ b/changelog.d/6690.bugfix
@@ -0,0 +1 @@
+Fix a bug where we would assign a numeric userid if somebody tried registering with an empty username.
diff --git a/synapse/handlers/register.py b/synapse/handlers/register.py
index 099611d79e..ba7323c74e 100644
--- a/synapse/handlers/register.py
+++ b/synapse/handlers/register.py
@@ -177,7 +177,7 @@ class RegistrationHandler(BaseHandler):
         if password:
             password_hash = yield self._auth_handler.hash(password)
 
-        if localpart:
+        if localpart is not None:
             yield self.check_username(localpart, guest_access_token=guest_access_token)
 
             was_guest = guest_access_token is not None