diff --git a/lib/tunnelbroker/tunnelbroker-context.js b/lib/tunnelbroker/tunnelbroker-context.js
--- a/lib/tunnelbroker/tunnelbroker-context.js
+++ b/lib/tunnelbroker/tunnelbroker-context.js
@@ -53,7 +53,7 @@
   const listeners = React.useRef<Set<TunnelbrokerSocketListener>>(new Set());
   const socket = React.useRef<?WebSocket>(null);
   const promises = React.useRef<Promises>({});
-  const heartbeatTimeoutID = React.useRef();
+  const heartbeatTimeoutID = React.useRef<?TimeoutID>();
 
   const stopHeartbeatTimeout = React.useCallback(() => {
     if (heartbeatTimeoutID.current) {
diff --git a/lib/types/tunnelbroker/connection-initialization-response-types.js b/lib/types/tunnelbroker/connection-initialization-response-types.js
--- a/lib/types/tunnelbroker/connection-initialization-response-types.js
+++ b/lib/types/tunnelbroker/connection-initialization-response-types.js
@@ -5,13 +5,18 @@
 
 import { tShape, tString } from '../../utils/validation-utils.js';
 
+type ConnectionInitializationSuccessStatus = { +type: 'Success' };
+type ConnectionInitializationErrorStatus = { +type: 'Error', +data: string };
 export type ConnectionInitializationStatus =
-  | { +type: 'Success' }
-  | { +type: 'Error', +data: string };
+  | ConnectionInitializationSuccessStatus
+  | ConnectionInitializationErrorStatus;
 
 const connectionInitializationStatusValidator = t.union([
-  tShape({ type: tString('Success') }),
-  tShape({ type: tString('Error'), data: t.String }),
+  tShape<ConnectionInitializationSuccessStatus>({ type: tString('Success') }),
+  tShape<ConnectionInitializationErrorStatus>({
+    type: tString('Error'),
+    data: t.String,
+  }),
 ]);
 
 export type ConnectionInitializationResponse = {
diff --git a/lib/types/tunnelbroker/message-to-device-request-status-types.js b/lib/types/tunnelbroker/message-to-device-request-status-types.js
--- a/lib/types/tunnelbroker/message-to-device-request-status-types.js
+++ b/lib/types/tunnelbroker/message-to-device-request-status-types.js
@@ -15,17 +15,33 @@
   error: t.String,
 });
 
+type MessageSentSuccessStatus = { +type: 'Success', +data: string };
+type MessageSentErrorStatus = { +type: 'Error', +data: Failure };
+type MessageSentInvalidRequestStatus = { +type: 'InvalidRequest' };
+type MessageSentSerializationErrorStatus = {
+  +type: 'SerializationError',
+  +data: string,
+};
 export type MessageSentStatus =
-  | { +type: 'Success', +data: string }
-  | { +type: 'Error', +data: Failure }
-  | { +type: 'InvalidRequest' }
-  | { +type: 'SerializationError', +data: string };
+  | MessageSentSuccessStatus
+  | MessageSentErrorStatus
+  | MessageSentInvalidRequestStatus
+  | MessageSentSerializationErrorStatus;
 
 const messageSentStatusValidator = t.union([
-  tShape({ type: tString('Success'), data: t.String }),
-  tShape({ type: tString('Error'), data: failureValidator }),
-  tShape({ type: tString('InvalidRequest') }),
-  tShape({ type: tString('SerializationError'), data: t.String }),
+  tShape<MessageSentSuccessStatus>({
+    type: tString('Success'),
+    data: t.String,
+  }),
+  tShape<MessageSentErrorStatus>({
+    type: tString('Error'),
+    data: failureValidator,
+  }),
+  tShape<MessageSentInvalidRequestStatus>({ type: tString('InvalidRequest') }),
+  tShape<MessageSentSerializationErrorStatus>({
+    type: tString('SerializationError'),
+    data: t.String,
+  }),
 ]);
 
 export type MessageToDeviceRequestStatus = {