diff --git a/ZENOH_FLAT_TRANSITION.md b/ZENOH_FLAT_TRANSITION.md
index 492d969a..f3b79d31 100644
--- a/ZENOH_FLAT_TRANSITION.md
+++ b/ZENOH_FLAT_TRANSITION.md
@@ -27,6 +27,7 @@ zenoh (Rust)
 | --- | --- | --- |
 | [#481](https://github.com/eclipse-zenoh/zenoh-java/pull/481) | Use receiver-style zenoh-flat-jni bindings (generated Session/Query/Publisher methods, split key-expr overloads, de-prefixed callback names); +61–83% subscriber throughput | merged |
 | [#484](https://github.com/eclipse-zenoh/zenoh-java/pull/484) | `Encoding`/`ZenohId.toString` become pure JVM values (fixes a per-message native leak); the conversion logic lives in the **shared tier** (`EncodingCodec`/`zidString` in zenoh-flat-jni, correspondence-tested against native, reusable by zenoh-kotlin) with only a thin facade here; publisher **default encoding set natively at declare** (plain puts cross no encoding data). Pairs with [zenoh-flat-jni#4](https://github.com/ZettaScaleLabs/zenoh-flat-jni/pull/4), [zenoh-flat#3](https://github.com/ZettaScaleLabs/zenoh-flat/pull/3) (merged), [prebindgen#80](https://github.com/milyin/prebindgen/pull/80) (merged) | in progress |
+| [#491](https://github.com/eclipse-zenoh/zenoh-java/pull/491) | `Session.undeclare` detaches the key-expr handle even when the native undeclare fails (the generated wrapper consumes it either way); found on the zenoh-kotlin port ([zenoh-kotlin#668](https://github.com/eclipse-zenoh/zenoh-kotlin/pull/668)) | open |
 
 Companion PRs in the upstream repos are coordinated per constituent PR (e.g.
 [ZettaScaleLabs/zenoh-flat-jni#3](https://github.com/ZettaScaleLabs/zenoh-flat-jni/pull/3)
diff --git a/zenoh-java/src/commonMain/kotlin/io/zenoh/Session.kt b/zenoh-java/src/commonMain/kotlin/io/zenoh/Session.kt
index 1101ece5..b28291c2 100644
--- a/zenoh-java/src/commonMain/kotlin/io/zenoh/Session.kt
+++ b/zenoh-java/src/commonMain/kotlin/io/zenoh/Session.kt
@@ -421,10 +421,15 @@ class Session private constructor(private val config: Config) : AutoCloseable {
         if (handle == null || handle.isClosed()) {
             throw ZError("Attempting to undeclare a non declared key expression.")
         }
-        run {
+        try {
             zSession.undeclareKeyexpr(handle, throwZError)
+        } finally {
+            // The generated wrapper consumes the handle even when the native
+            // undeclare fails (the Rust side takes it by value) — detach it
+            // either way, degrading the KeyExpr to its string form instead of
+            // leaving a dead handle to be selected by later operations.
+            keyExpr.handle = null
         }
-        keyExpr.handle = null
     }
 
     /**
diff --git a/zenoh-java/src/jvmTest/kotlin/io/zenoh/KeyExprHandleTest.kt b/zenoh-java/src/jvmTest/kotlin/io/zenoh/KeyExprHandleTest.kt
index afae4ab3..e9a3c2e0 100644
--- a/zenoh-java/src/jvmTest/kotlin/io/zenoh/KeyExprHandleTest.kt
+++ b/zenoh-java/src/jvmTest/kotlin/io/zenoh/KeyExprHandleTest.kt
@@ -15,6 +15,7 @@
 package io.zenoh
 
 import io.zenoh.bytes.ZBytes
+import io.zenoh.exceptions.ZError
 import io.zenoh.keyexpr.KeyExpr
 import io.zenoh.sample.Sample
 import org.junit.Assert.assertEquals
@@ -62,6 +63,30 @@ class KeyExprHandleTest {
         session.close()
     }
 
+    @Test
+    fun failedUndeclarationStillDetachesTheConsumedHandle() {
+        // Undeclaring through the WRONG session makes the native undeclare
+        // fail — and the generated wrapper consumes the handle even then. The
+        // KeyExpr must degrade to its string form (handle detached), not keep
+        // selecting a dead handle.
+        val session1 = Zenoh.open(Config.loadDefault())
+        val session2 = Zenoh.open(Config.loadDefault())
+        val declared = session1.declareKeyExpr("example/testing/keyexpr/wrongsession")
+        var failed = false
+        try {
+            session2.undeclare(declared)
+        } catch (e: ZError) {
+            failed = true
+        }
+        assertTrue(failed)
+        assertNull(declared.handle)
+        // String-backed operation keeps working after the failed undeclare.
+        assertTrue(declared.intersects(KeyExpr.tryFrom("example/testing/**")))
+        session1.put(declared, ZBytes.from("test"))
+        session1.close()
+        session2.close()
+    }
+
     @Test
     fun receivedKeyExprIsStringBackedAndReusable() {
         val session = Zenoh.open(Config.loadDefault())