diff --git a/data/web/modals/admin.php b/data/web/modals/admin.php
index 4b090999..2897e23b 100644
--- a/data/web/modals/admin.php
+++ b/data/web/modals/admin.php
@@ -59,8 +59,8 @@ if (!isset($_SESSION['mailcow_cc_role'])) {
             <div class="form-group">
               <label class="control-label col-sm-2" for="username"><?=$lang['admin']['username'];?>:</label>
               <div class="col-sm-10">
-                <input type="text" class="form-control" name="username" required>
-                &rdsh; <kbd>a-z A-Z - _ .</kbd>
+                <input type="text" class="form-control" name="username" onkeyup="this.value = this.value.toLowerCase();" required>
+                &rdsh; <kbd>a-z - _ .</kbd>
               </div>
             </div>
             <div class="form-group">
@@ -171,8 +171,8 @@ if (!isset($_SESSION['mailcow_cc_role'])) {
             <div class="form-group">
               <label class="control-label col-sm-2" for="username"><?=$lang['admin']['username'];?>:</label>
               <div class="col-sm-10">
-                <input type="text" class="form-control" name="username" required>
-                &rdsh; <kbd>a-z A-Z - _ .</kbd>
+                <input type="text" class="form-control" name="username" onkeyup="this.value = this.value.toLowerCase();" required>
+                &rdsh; <kbd>a-z - _ .</kbd>
               </div>
             </div>
             <div class="form-group">