diff --git a/src/components/views/auth/RegistrationForm.js b/src/components/views/auth/RegistrationForm.js
index ccbfc507c6..ad663bee47 100644
--- a/src/components/views/auth/RegistrationForm.js
+++ b/src/components/views/auth/RegistrationForm.js
@@ -384,7 +384,7 @@ module.exports = React.createClass({
     },
 
     validateUsernameRules: withValidation({
-        description: () => _t("Use letters, numbers, dashes and underscores only"),
+        description: () => _t("Use lowercase letters, numbers, dashes and underscores only"),
         rules: [
             {
                 key: "required",
diff --git a/src/i18n/strings/en_EN.json b/src/i18n/strings/en_EN.json
index 1e88307533..bd4f3820b0 100644
--- a/src/i18n/strings/en_EN.json
+++ b/src/i18n/strings/en_EN.json
@@ -1360,7 +1360,7 @@
     "Other users can invite you to rooms using your contact details": "Other users can invite you to rooms using your contact details",
     "Enter phone number (required on this homeserver)": "Enter phone number (required on this homeserver)",
     "Doesn't look like a valid phone number": "Doesn't look like a valid phone number",
-    "Use letters, numbers, dashes and underscores only": "Use letters, numbers, dashes and underscores only",
+    "Use lowercase letters, numbers, dashes and underscores only": "Use lowercase letters, numbers, dashes and underscores only",
     "Enter username": "Enter username",
     "Some characters not allowed": "Some characters not allowed",
     "Email (optional)": "Email (optional)",