diff --git a/src/schemas.ts b/src/schemas.ts
index b9630ba..4e62f96 100644
--- a/src/schemas.ts
+++ b/src/schemas.ts
@@ -3,7 +3,9 @@ import * as Yup from "yup";
 const REQUIRED = "Required.";
 const USERNAME = Yup.string()
   .required(REQUIRED)
-  .min(1, "Username is too short.");
+  .min(1, "Username is too short.")
+  .matches(/^[a-z0-9]+$/, "Username must be lowercase letters and numbers.");
+
 const DISPLAY_NAME = Yup.string()
   .required(REQUIRED)
   .min(1, "Display name is too short.");