diff --git a/gui.py b/gui.py
index 4d736c5..a071442 100644
--- a/gui.py
+++ b/gui.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import re
 import sys
 import ctypes
 import asyncio
@@ -474,7 +475,10 @@ class LoginForm:
                 self._token_entry.get().strip(),
             )
             # basic input data validation
-            if len(login_data.username) < 3:
+            if (
+                3 <= len(login_data.username) <= 25  # 3-25 characters in length
+                and re.match(r'^[a-zA-Z0-9_]+$', login_data.username)  # only ascii and underscores
+            ):
                 self.clear(login=True)
                 continue
             if len(login_data.password) < 8:
diff --git a/twitch.py b/twitch.py
index fc21501..0e0c420 100644
--- a/twitch.py
+++ b/twitch.py
@@ -330,9 +330,12 @@ class _AuthState:
                     logger.info("1000: CAPTCHA is required")
                     use_chrome = True
                     break
-                elif error_code == 3001:
+                elif error_code in (2004, 3001):
                     logger.info("3001: Login failed due to incorrect username or password")
                     gui_print(_("login", "incorrect_login_pass"))
+                    if error_code == 2004:
+                        # invalid username
+                        login_form.clear(login=True)
                     login_form.clear(password=True)
                     continue
                 elif error_code in (