diff --git a/src/editor/Editor.tsx b/src/editor/Editor.tsx
index 968c39d2..3033742e 100644
--- a/src/editor/Editor.tsx
+++ b/src/editor/Editor.tsx
@@ -225,7 +225,7 @@ const mapDispatchToProps: DispatchProps = {
     onProgramStorageChanged: storageChanged,
     // REVISIT: the options here might need to be changed - hopefully there is
     // one setting that works for all hub types for cases where we aren't connected.
-    onCheck: compile,
+    onCheck: (script) => compile(script, []),
     onToggleDocs: () => toggleBoolean(SettingId.ShowDocs),
 };
 
diff --git a/src/firmware/FlashButton.tsx b/src/firmware/FlashButton.tsx
index cba488ad..89e73804 100644
--- a/src/firmware/FlashButton.tsx
+++ b/src/firmware/FlashButton.tsx
@@ -28,7 +28,7 @@ const mapDispatchToProps: DispatchProps = {
     onFile: flashFirmware,
     onReject: (file) =>
         notification.add('error', `'${file.name}' is not a valid firmware file.`),
-    onClick: () => flashFirmware(),
+    onClick: () => flashFirmware(null),
 };
 
 const mergeProps = (
diff --git a/src/firmware/actions.ts b/src/firmware/actions.ts
index cda81bac..d02cd4e5 100644
--- a/src/firmware/actions.ts
+++ b/src/firmware/actions.ts
@@ -129,15 +129,15 @@ export type FailToFinishReason =
  * Action that flashes firmware to a hub.
  */
 export type FlashFirmwareFlashAction = Action<FlashFirmwareActionType.FlashFirmware> & {
-    /** The firmware zip file data or undefined to get firmware later. */
-    data?: ArrayBuffer;
+    /** The firmware zip file data or `null` to get firmware later. */
+    data: ArrayBuffer | null;
 };
 
 /**
  * Creates a new action to flash firmware to a hub.
- * @param data The firmware zip file data or undefined to get firmware later.
+ * @param data The firmware zip file data or `null` to get firmware later.
  */
-export function flashFirmware(data?: ArrayBuffer): FlashFirmwareFlashAction {
+export function flashFirmware(data: ArrayBuffer | null): FlashFirmwareFlashAction {
     return { type: FlashFirmwareActionType.FlashFirmware, data };
 }
 
diff --git a/src/firmware/sagas.test.ts b/src/firmware/sagas.test.ts
index e6504e4d..6ef5f873 100644
--- a/src/firmware/sagas.test.ts
+++ b/src/firmware/sagas.test.ts
@@ -88,7 +88,7 @@ describe('flashFirmware', () => {
 
             // saga is triggered by this action
 
-            saga.put(flashFirmwareAction());
+            saga.put(flashFirmwareAction(null));
 
             // first step is to connect to the hub bootloader
 
@@ -240,7 +240,7 @@ describe('flashFirmware', () => {
 
             // saga is triggered by this action
 
-            saga.put(flashFirmwareAction());
+            saga.put(flashFirmwareAction(null));
 
             // first step is to connect to the hub bootloader
 
@@ -296,7 +296,7 @@ describe('flashFirmware', () => {
 
             // saga is triggered by this action
 
-            saga.put(flashFirmwareAction());
+            saga.put(flashFirmwareAction(null));
 
             // first step is to connect to the hub bootloader
 
@@ -370,7 +370,7 @@ describe('flashFirmware', () => {
 
             // saga is triggered by this action
 
-            saga.put(flashFirmwareAction());
+            saga.put(flashFirmwareAction(null));
 
             // first step is to connect to the hub bootloader
 
@@ -440,7 +440,7 @@ describe('flashFirmware', () => {
 
             // saga is triggered by this action
 
-            saga.put(flashFirmwareAction());
+            saga.put(flashFirmwareAction(null));
 
             // first step is to connect to the hub bootloader
 
@@ -513,7 +513,7 @@ describe('flashFirmware', () => {
 
             // saga is triggered by this action
 
-            saga.put(flashFirmwareAction());
+            saga.put(flashFirmwareAction(null));
 
             // first step is to connect to the hub bootloader
 
@@ -579,7 +579,7 @@ describe('flashFirmware', () => {
 
             // saga is triggered by this action
 
-            saga.put(flashFirmwareAction());
+            saga.put(flashFirmwareAction(null));
 
             // first step is to connect to the hub bootloader
 
@@ -651,7 +651,7 @@ describe('flashFirmware', () => {
 
             // saga is triggered by this action
 
-            saga.put(flashFirmwareAction());
+            saga.put(flashFirmwareAction(null));
 
             // first step is to connect to the hub bootloader
 
@@ -739,7 +739,7 @@ describe('flashFirmware', () => {
 
             // saga is triggered by this action
 
-            saga.put(flashFirmwareAction());
+            saga.put(flashFirmwareAction(null));
 
             // first step is to connect to the hub bootloader
 
@@ -811,7 +811,7 @@ describe('flashFirmware', () => {
 
             // saga is triggered by this action
 
-            saga.put(flashFirmwareAction());
+            saga.put(flashFirmwareAction(null));
 
             // first step is to connect to the hub bootloader
 
@@ -912,7 +912,7 @@ describe('flashFirmware', () => {
 
             // saga is triggered by this action
 
-            saga.put(flashFirmwareAction());
+            saga.put(flashFirmwareAction(null));
 
             // first step is to connect to the hub bootloader
 
@@ -1022,7 +1022,7 @@ describe('flashFirmware', () => {
 
             // saga is triggered by this action
 
-            saga.put(flashFirmwareAction());
+            saga.put(flashFirmwareAction(null));
 
             // first step is to connect to the hub bootloader
 
@@ -1172,7 +1172,7 @@ describe('flashFirmware', () => {
 
             // saga is triggered by this action
 
-            saga.put(flashFirmwareAction());
+            saga.put(flashFirmwareAction(null));
 
             // first step is to connect to the hub bootloader
 
@@ -1854,7 +1854,7 @@ describe('flashFirmware', () => {
 
         // saga is triggered by this action
 
-        saga.put(flashFirmwareAction());
+        saga.put(flashFirmwareAction(null));
 
         // first step is to connect to the hub bootloader
 
diff --git a/src/firmware/sagas.ts b/src/firmware/sagas.ts
index a6e72b52..d3b36f60 100644
--- a/src/firmware/sagas.ts
+++ b/src/firmware/sagas.ts
@@ -277,7 +277,7 @@ function* flashFirmware(action: FlashFirmwareFlashAction): Generator {
             program = editor.getValue();
         }
 
-        if (action.data !== undefined) {
+        if (action.data !== null) {
             ({ firmware, deviceId } = yield* loadFirmware(action.data, program));
         }
 
diff --git a/src/lwp3-bootloader/actions.ts b/src/lwp3-bootloader/actions.ts
index 6efa0d25..c8ea49b6 100644
--- a/src/lwp3-bootloader/actions.ts
+++ b/src/lwp3-bootloader/actions.ts
@@ -124,13 +124,13 @@ export function didFailToConnect(
 
 export function didFailToConnect(
     reason: BootloaderConnectionFailureReason,
-    err?: Error,
+    arg1?: Error,
 ): BootloaderConnectionDidFailToConnectAction {
     if (reason === BootloaderConnectionFailureReason.Unknown) {
         return <BootloaderConnectionDidFailToConnectAction>{
             type: BootloaderConnectionActionType.DidFailToConnect,
             reason,
-            err,
+            err: arg1,
         };
     }
     return { type: BootloaderConnectionActionType.DidFailToConnect, reason };
diff --git a/src/mpy/actions.ts b/src/mpy/actions.ts
index 7663f295..b84e8240 100644
--- a/src/mpy/actions.ts
+++ b/src/mpy/actions.ts
@@ -14,10 +14,10 @@ export type MpyCompileAction = Action<MpyActionType.Compile> & {
     /** The script to compile. */
     readonly script: string;
     /** The compiler command line options */
-    options?: string[];
+    options: string[];
 };
 
-export function compile(script: string, options?: string[]): MpyCompileAction {
+export function compile(script: string, options: string[]): MpyCompileAction {
     return { type: MpyActionType.Compile, script, options };
 }
 
diff --git a/src/mpy/sagas.test.ts b/src/mpy/sagas.test.ts
index 6a9fa18d..57878652 100644
--- a/src/mpy/sagas.test.ts
+++ b/src/mpy/sagas.test.ts
@@ -18,7 +18,7 @@ enum MpyFeatureFlags {
 test('compiler works', async () => {
     const saga = new AsyncSaga(mpy);
 
-    saga.put(compile('print("hello!")'));
+    saga.put(compile('print("hello!")', []));
 
     const action = await saga.take();
     expect(action.type).toBe(MpyActionType.DidCompile);
@@ -32,7 +32,7 @@ test('compiler works', async () => {
 test('compiler error works', async () => {
     const saga = new AsyncSaga(mpy);
 
-    saga.put(compile('syntax error!'));
+    saga.put(compile('syntax error!', []));
 
     const action = await saga.take();
     expect(action.type).toBe(MpyActionType.DidFailToCompile);