diff --git a/receivers/common/web/ConnectionMonitor.ts b/receivers/common/web/ConnectionMonitor.ts
index 26e09e4..9858ff1 100644
--- a/receivers/common/web/ConnectionMonitor.ts
+++ b/receivers/common/web/ConnectionMonitor.ts
@@ -70,6 +70,11 @@ export class ConnectionMonitor {
                             listener.send(Opcode.Ping, null, sessionId);
                             ConnectionMonitor.heartbeatRetries.set(sessionId, ConnectionMonitor.heartbeatRetries.get(sessionId) + 1);
                         }
+                        else if (listener.getSessionProtocolVersion(sessionId) === undefined) {
+                            ConnectionMonitor.logger.warn(`Session ${sessionId} was not found in the list of active sessions. Removing...`);
+                            ConnectionMonitor.backendConnections.delete(sessionId);
+                            ConnectionMonitor.heartbeatRetries.delete(sessionId);
+                        }
                     }
                 }
             }, ConnectionMonitor.connectionPingTimeout);
diff --git a/receivers/common/web/ListenerService.ts b/receivers/common/web/ListenerService.ts
index 16dbb85..4d0a1e9 100644
--- a/receivers/common/web/ListenerService.ts
+++ b/receivers/common/web/ListenerService.ts
@@ -82,7 +82,7 @@ export abstract class ListenerService {
     }
 
     public getSessionProtocolVersion(sessionId: string) {
-        return this.sessionMap.get(sessionId).protocolVersion;
+        return this.sessionMap.get(sessionId)?.protocolVersion;
     }
 
     private isSubscribedToEvent(sessionId: string, event: EventSubscribeObject) {