diff --git a/src/components/views/dialogs/SetMxIdDialog.js b/src/components/views/dialogs/SetMxIdDialog.js
index 4d4f672f2b..554a244358 100644
--- a/src/components/views/dialogs/SetMxIdDialog.js
+++ b/src/components/views/dialogs/SetMxIdDialog.js
@@ -106,6 +106,16 @@ export default React.createClass({
     },
 
     _doUsernameCheck: function() {
+        // XXX: SPEC-1
+        // Check if username is valid
+        // Naive impl copied from https://github.com/matrix-org/matrix-react-sdk/blob/66c3a6d9ca695780eb6b662e242e88323053ff33/src/components/views/login/RegistrationForm.js#L190
+        if (encodeURIComponent(this.state.username) !== this.state.username) {
+            this.setState({
+                usernameError: _t('User names may only contain letters, numbers, dots, hyphens and underscores.'),
+            });
+            return Promise.reject();
+        }
+
         // Check if username is available
         return this._matrixClient.isUsernameAvailable(this.state.username).then(
             (isAvailable) => {
@@ -242,7 +252,7 @@ export default React.createClass({
         return (
             <BaseDialog className="mx_SetMxIdDialog"
                 onFinished={this.props.onFinished}
-                title="To get started, please pick a username!"
+                title={_t('To get started, please pick a username!')}
             >
                 <div className="mx_Dialog_content">
                     <div className="mx_SetMxIdDialog_input_group">
diff --git a/src/i18n/strings/en_EN.json b/src/i18n/strings/en_EN.json
index 0d5b7d9d96..5381fc2356 100644
--- a/src/i18n/strings/en_EN.json
+++ b/src/i18n/strings/en_EN.json
@@ -575,6 +575,7 @@
     "To configure the room": "To configure the room",
     "to demote": "to demote",
     "to favourite": "to favourite",
+    "To get started, please pick a username!": "To get started, please pick a username!",
     "To invite users into the room": "To invite users into the room",
     "To kick users": "To kick users",
     "To link to a room it must have <a>an address</a>.": "To link to a room it must have <a>an address</a>.",