diff --git a/telethon/utils.py b/telethon/utils.py
index 643fa746..607897e4 100644
--- a/telethon/utils.py
+++ b/telethon/utils.py
@@ -30,6 +30,8 @@ USERNAME_RE = re.compile(
     r'@|(?:https?://)?(?:telegram\.(?:me|dog)|t\.me)/(joinchat/)?'
 )
 
+VALID_USERNAME_RE = re.compile(r'^[a-zA-Z][\w\d]{3,30}[a-zA-Z\d]$')
+
 
 def get_display_name(entity):
     """Gets the input peer for the given "entity" (user, chat or channel)
@@ -326,6 +328,7 @@ def is_image(file):
     return (isinstance(file, str) and
             (mimetypes.guess_type(file)[0] or '').startswith('image/'))
 
+
 def is_audio(file):
     """Returns True if the file extension looks like an audio file"""
     return (isinstance(file, str) and
@@ -353,15 +356,21 @@ def parse_username(username):
        a string, username or URL. Returns a tuple consisting of
        both the stripped, lowercase username and whether it is
        a joinchat/ hash (in which case is not lowercase'd).
+
+       Returns None if the username is not valid.
     """
     username = username.strip()
     m = USERNAME_RE.match(username)
     if m:
-        result = username[m.end():]
+        username = username[m.end():]
         is_invite = bool(m.group(1))
-        return result if is_invite else result.lower(), is_invite
-    else:
+        if is_invite:
+            return username, True
+
+    if VALID_USERNAME_RE.match(username):
         return username.lower(), False
+    else:
+        return None, False
 
 
 def get_peer_id(peer):