diff --git a/node-api/src/main/kotlin/net/corda/nodeapi/internal/SignedNodeInfo.kt b/node-api/src/main/kotlin/net/corda/nodeapi/internal/SignedNodeInfo.kt
index c0303c28ee..0b8657600b 100644
--- a/node-api/src/main/kotlin/net/corda/nodeapi/internal/SignedNodeInfo.kt
+++ b/node-api/src/main/kotlin/net/corda/nodeapi/internal/SignedNodeInfo.kt
@@ -35,7 +35,7 @@ class SignedNodeInfo(val raw: SerializedBytes<NodeInfo>, val signatures: List<Di
     fun verified(): NodeInfo {
         val nodeInfo = raw.deserialize()
         val identities = nodeInfo.legalIdentities.filterNot { it.owningKey is CompositeKey }
-
+        require(identities.isNotEmpty()) { "At least one identity with a non-composite key needs to be specified." }
         if (identities.size < signatures.size) {
             throw SignatureException("Extra signatures. Found ${signatures.size} expected ${identities.size}")
         }
diff --git a/node/src/main/kotlin/net/corda/node/services/messaging/RPCServer.kt b/node/src/main/kotlin/net/corda/node/services/messaging/RPCServer.kt
index de817f0491..320d639664 100644
--- a/node/src/main/kotlin/net/corda/node/services/messaging/RPCServer.kt
+++ b/node/src/main/kotlin/net/corda/node/services/messaging/RPCServer.kt
@@ -18,9 +18,6 @@ import com.esotericsoftware.kryo.io.Output
 import com.github.benmanes.caffeine.cache.Cache
 import com.github.benmanes.caffeine.cache.Caffeine
 import com.github.benmanes.caffeine.cache.RemovalListener
-import com.google.common.collect.HashMultimap
-import com.google.common.collect.Multimaps
-import com.google.common.collect.SetMultimap
 import com.google.common.util.concurrent.ThreadFactoryBuilder
 import net.corda.client.rpc.RPCException
 import net.corda.core.context.Actor
@@ -122,7 +119,7 @@ class RPCServer(
     /** The observable subscription mapping. */
     private val observableMap = createObservableSubscriptionMap()
     /** A mapping from client addresses to IDs of associated Observables */
-    private val clientAddressToObservables = Multimaps.synchronizedSetMultimap(HashMultimap.create<SimpleString, InvocationId>())
+    private val clientAddressToObservables = ConcurrentHashMap<SimpleString, HashSet<InvocationId>>()
     /** The scheduled reaper handle. */
     private var reaperScheduledFuture: ScheduledFuture<*>? = null
 
@@ -301,8 +298,10 @@ class RPCServer(
     // Observables may be serialised and thus registered.
     private fun invalidateClient(clientAddress: SimpleString) {
         lifeCycle.requireState(State.STARTED)
-        val observableIds = clientAddressToObservables.removeAll(clientAddress)
-        observableMap.invalidateAll(observableIds)
+        val observableIds = clientAddressToObservables.remove(clientAddress)
+        if (observableIds != null) {
+            observableMap.invalidateAll(observableIds)
+        }
         responseMessageBuffer.remove(clientAddress)
     }
 
@@ -429,7 +428,7 @@ class RPCServer(
      */
     inner class ObservableContext(
             val observableMap: ObservableSubscriptionMap,
-            val clientAddressToObservables: SetMultimap<SimpleString, InvocationId>,
+            val clientAddressToObservables: ConcurrentHashMap<SimpleString, HashSet<InvocationId>>,
             val deduplicationIdentity: String,
             val clientAddress: SimpleString
     ) {
@@ -535,11 +534,30 @@ object RpcServerObservableSerializer : Serializer<Observable<*>>() {
                             }
 
                             override fun onCompleted() {
+                                observableContext.clientAddressToObservables.compute(observableContext.clientAddress) { _, observables ->
+                                    if (observables != null) {
+                                        observables.remove(observableId)
+                                        if (observables.isEmpty()) {
+                                            null
+                                        } else {
+                                            observables
+                                        }
+                                    } else {
+                                        null
+                                    }
+                                }
                             }
                         }
                 )
         )
-        observableContext.clientAddressToObservables.put(observableContext.clientAddress, observableId)
+        observableContext.clientAddressToObservables.compute(observableContext.clientAddress) { _, observables ->
+            if (observables == null) {
+                hashSetOf(observableId)
+            } else {
+                observables.add(observableId)
+                observables
+            }
+        }
         observableContext.observableMap.put(observableId, observableWithSubscription)
     }