diff --git a/okhttp/src/commonJvmAndroid/kotlin/okhttp3/internal/connection/RouteSelector.kt b/okhttp/src/commonJvmAndroid/kotlin/okhttp3/internal/connection/RouteSelector.kt
index 10622d9e6260..04493b4cbc8a 100644
--- a/okhttp/src/commonJvmAndroid/kotlin/okhttp3/internal/connection/RouteSelector.kt
+++ b/okhttp/src/commonJvmAndroid/kotlin/okhttp3/internal/connection/RouteSelector.kt
@@ -221,7 +221,14 @@ class RouteSelector internal constructor(
         else -> socketPort
       }
 
-    val dnsRequest = Dns.Request(socketHost, dnsPort)
+    val dnsRequest =
+      try {
+        Dns.Request(socketHost, dnsPort)
+      } catch (e: IllegalArgumentException) {
+        throw UnknownHostException("invalid host $socketHost.").apply {
+          initCause(e)
+        }
+      }
     val result =
       when (val dnsCall = address.dns.newCall(dnsRequest)) {
         is LookupDnsCall -> {
diff --git a/okhttp/src/jvmTest/kotlin/okhttp3/CallTest.kt b/okhttp/src/jvmTest/kotlin/okhttp3/CallTest.kt
index e00c97a4e8e1..b2bb2351dfd6 100644
--- a/okhttp/src/jvmTest/kotlin/okhttp3/CallTest.kt
+++ b/okhttp/src/jvmTest/kotlin/okhttp3/CallTest.kt
@@ -45,6 +45,7 @@ import java.net.CookiePolicy
 import java.net.HttpCookie
 import java.net.HttpURLConnection
 import java.net.InetAddress
+import java.net.InetSocketAddress
 import java.net.ProtocolException
 import java.net.Proxy
 import java.net.SocketTimeoutException
@@ -1072,6 +1073,21 @@ open class CallTest {
       }
   }
 
+  /** https://github.com/lysine-dev/okhttp/issues/9758 */
+  @Test
+  fun malformedProxyHostnameFailsGracefully() {
+    val proxySelector = RecordingProxySelector()
+    proxySelector.proxies.add(
+      Proxy(
+        Proxy.Type.HTTP,
+        InetSocketAddress.createUnresolved("internet ", 8080),
+      ),
+    )
+    client = client.newBuilder().proxySelector(proxySelector).build()
+    executeSynchronously(server.url("/").toString())
+      .assertFailure(UnknownHostException::class.java)
+  }
+
   /** https://github.com/lysine-dev/okhttp/issues/4875  */
   @Test
   fun interceptorRecoversWhenRoutesExhausted() {
diff --git a/okhttp/src/jvmTest/kotlin/okhttp3/internal/connection/RouteSelectorTest.kt b/okhttp/src/jvmTest/kotlin/okhttp3/internal/connection/RouteSelectorTest.kt
index 38c4e52b30c5..b3357cac1c8b 100644
--- a/okhttp/src/jvmTest/kotlin/okhttp3/internal/connection/RouteSelectorTest.kt
+++ b/okhttp/src/jvmTest/kotlin/okhttp3/internal/connection/RouteSelectorTest.kt
@@ -50,6 +50,7 @@ import org.junit.jupiter.api.AfterEach
 import org.junit.jupiter.api.Assumptions.assumeTrue
 import org.junit.jupiter.api.BeforeEach
 import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.assertThrows
 import org.junit.jupiter.api.extension.RegisterExtension
 
 @Burst
@@ -479,6 +480,20 @@ class RouteSelectorTest(
     assertThat(routeSelector.hasNext()).isFalse()
   }
 
+  @Test fun failedRouteWithInvalidProxy() {
+    val address =
+      factory.newAddress(
+        proxy =
+          Proxy(
+            Proxy.Type.HTTP,
+            InetSocketAddress.createUnresolved("a b", 8080),
+          ),
+      )
+    val routeSelector = newRouteSelector(address)
+
+    assertThrows<UnknownHostException> { routeSelector.next() }
+  }
+
   @Test fun queryForAllSelectedRoutes() {
     val address = factory.newAddress()
     val routeSelector = newRouteSelector(address)