diff --git a/src/aioweb/session.py b/src/aioweb/session.py
index e0501f7..d1c725f 100644
--- a/src/aioweb/session.py
+++ b/src/aioweb/session.py
@@ -188,17 +188,19 @@ class ExtendedSession:
     def overwrite_domain(self, target, replacement):
         """register a host-substring rewrite (target -> replacement)
 
-        raises:
-            ValueError: if replacement contains a port (e.g. 'host:1234') - yarl's
-                with_host() rejects any ':' in a host at request time, deep inside
-                yarl with a stack trace far from this call; caught here instead
+        raises ValueError if the replacement is not a valid host for yarl's with_host()
+        (e.g. a 'host:port' string or a pre-bracketed '[::1]') - caught here rather than
+        deep in yarl at request time. a bare IPv6 literal ('::1', '2001:db8::1') is valid
+        and accepted: yarl brackets it itself.
         """
-        if ":" in replacement:
+        try:
+            URL("http://placeholder").with_host(replacement)
+        except ValueError as exc:
             raise ValueError(
-                f"overwrite_domain replacement {replacement!r} contains a port - "
-                "yarl's URL.with_host() cannot take a host:port string; rewrite the "
-                "port separately or via a full URL override, not domain_overwrites"
-            )
+                f"overwrite_domain replacement {replacement!r} is not a valid host: {exc}; "
+                "pass a bare hostname or IP (a bare IPv6 literal is fine), not a host:port or "
+                "bracketed form - rewrite a port via a full URL override, not domain_overwrites"
+            ) from exc
         self.domain_overwrites[target] = replacement
 
     def _apply_domain_overwrites(self, url: str) -> str: