diff --git a/src/components/views/login/PasswordLogin.js b/src/components/views/login/PasswordLogin.js
index 2c125be378..9f855616fc 100644
--- a/src/components/views/login/PasswordLogin.js
+++ b/src/components/views/login/PasswordLogin.js
@@ -71,7 +71,7 @@ class PasswordLogin extends React.Component {
         ev.preventDefault();
         if (this.state.loginType === PasswordLogin.LOGIN_FIELD_PHONE) {
             this.props.onSubmit(
-                '',
+                '', // XXX: Synapse breaks if you send null here:
                 this.state.phoneCountry,
                 this.state.phoneNumber,
                 this.state.password,