diff --git a/node/src/main/kotlin/com/r3corda/node/services/network/InMemoryNetworkMapCache.kt b/node/src/main/kotlin/com/r3corda/node/services/network/InMemoryNetworkMapCache.kt
index e7438ccff9..4fe8d4f911 100644
--- a/node/src/main/kotlin/com/r3corda/node/services/network/InMemoryNetworkMapCache.kt
+++ b/node/src/main/kotlin/com/r3corda/node/services/network/InMemoryNetworkMapCache.kt
@@ -6,6 +6,7 @@ import com.google.common.util.concurrent.SettableFuture
 import com.r3corda.core.bufferUntilSubscribed
 import com.r3corda.core.contracts.Contract
 import com.r3corda.core.crypto.Party
+import com.r3corda.core.crypto.toStringShort
 import com.r3corda.core.map
 import com.r3corda.core.messaging.MessagingService
 import com.r3corda.core.messaging.SingleMessageRecipient
@@ -72,9 +73,16 @@ open class InMemoryNetworkMapCache : SingletonSerializeAsToken(), NetworkMapCach
     override fun get(serviceType: ServiceType) = registeredNodes.filterValues { it.advertisedServices.any { it.info.type.isSubTypeOf(serviceType) } }.map { it.value }
     override fun getRecommended(type: ServiceType, contract: Contract, vararg party: Party): NodeInfo? = get(type).firstOrNull()
     override fun getNodeByLegalName(name: String) = get().singleOrNull { it.legalIdentity.name == name }
-    override fun getNodeByPublicKey(publicKey: PublicKey) = get().singleOrNull {
-        (it.legalIdentity.owningKey == publicKey)
-            || it.advertisedServices.any { it.identity.owningKey == publicKey }
+    override fun getNodeByPublicKey(publicKey: PublicKey): NodeInfo? {
+        // Although we should never have more than one match, it is theoretically possible. Report an error if it happens.
+        val candidates = get().filter {
+            (it.legalIdentity.owningKey == publicKey)
+                    || it.advertisedServices.any { it.identity.owningKey == publicKey }
+        }
+        if (candidates.size > 1) {
+            throw IllegalStateException("Found more than one match for key ${publicKey.toStringShort()}")
+        }
+        return candidates.singleOrNull()
     }
 
     override fun addMapService(net: MessagingService, networkMapAddress: SingleMessageRecipient, subscribe: Boolean,
diff --git a/node/src/test/kotlin/com/r3corda/node/services/InMemoryNetworkMapCacheTest.kt b/node/src/test/kotlin/com/r3corda/node/services/InMemoryNetworkMapCacheTest.kt
index 02087ccd1c..3fa1062f05 100644
--- a/node/src/test/kotlin/com/r3corda/node/services/InMemoryNetworkMapCacheTest.kt
+++ b/node/src/test/kotlin/com/r3corda/node/services/InMemoryNetworkMapCacheTest.kt
@@ -1,8 +1,14 @@
 package com.r3corda.node.services
 
+import com.r3corda.core.crypto.generateKeyPair
+import com.r3corda.core.node.services.ServiceInfo
+import com.r3corda.node.services.network.NetworkMapService
+import com.r3corda.node.services.transactions.SimpleNotaryService
+import com.r3corda.testing.expect
 import com.r3corda.testing.node.MockNetwork
 import org.junit.Before
 import org.junit.Test
+import kotlin.test.assertEquals
 
 class InMemoryNetworkMapCacheTest {
     lateinit var network: MockNetwork
@@ -20,4 +26,20 @@ class InMemoryNetworkMapCacheTest {
         network.runNetwork()
         future.get()
     }
+
+    @Test
+    fun `key collision`() {
+        val keyPair = generateKeyPair()
+        val nodeA = network.createNode(null, -1, MockNetwork.DefaultFactory, true, "Node A", keyPair, ServiceInfo(NetworkMapService.type))
+        val nodeB = network.createNode(null, -1, MockNetwork.DefaultFactory, true, "Node B", keyPair, ServiceInfo(NetworkMapService.type))
+
+        // Node A currently knows only about itself, so this returns node A
+        assertEquals(nodeA.netMapCache.getNodeByPublicKey(keyPair.public), nodeA.info)
+
+        nodeA.netMapCache.addNode(nodeB.info)
+        // Now both nodes match, so it throws an error
+        expect<IllegalStateException> {
+            nodeA.netMapCache.getNodeByPublicKey(keyPair.public)
+        }
+    }
 }