diff --git a/tests/cts/net/src/android/net/cts/EthernetManagerTest.kt b/tests/cts/net/src/android/net/cts/EthernetManagerTest.kt
index e865480..882bbb9 100644
--- a/tests/cts/net/src/android/net/cts/EthernetManagerTest.kt
+++ b/tests/cts/net/src/android/net/cts/EthernetManagerTest.kt
@@ -312,10 +312,12 @@
         private val result = CompletableFuture<String>()
 
         override fun onResult(iface: String) {
+            assertFalse(result.isDone())
             result.complete(iface)
         }
 
         override fun onError(e: EthernetNetworkManagementException) {
+            assertFalse(result.isDone())
             result.completeExceptionally(e)
         }