diff --git a/ui/src/pages/vendor/CreateTenantPage.tsx b/ui/src/pages/vendor/CreateTenantPage.tsx
index d015548..5c1bad7 100644
--- a/ui/src/pages/vendor/CreateTenantPage.tsx
+++ b/ui/src/pages/vendor/CreateTenantPage.tsx
@@ -91,11 +91,11 @@ export function CreateTenantPage() {
             </select>
           </FormField>
 
-          <FormField label="Admin Username" htmlFor="admin-user" hint="Initial tenant admin (owner role)">
+          <FormField label="Admin Username" htmlFor="admin-user" hint="Initial tenant admin (owner role). Alphanumeric only, no hyphens.">
             <Input
               id="admin-user"
               value={adminUsername}
-              onChange={(e) => setAdminUsername(e.target.value)}
+              onChange={(e) => setAdminUsername(e.target.value.replace(/[^a-zA-Z0-9]/g, ''))}
               placeholder="admin"
             />
           </FormField>