diff --git a/packages/app-testing/src/CallDisplay/Error.js b/packages/app-testing/src/CallDisplay/Error.js
index b596981001..1c76b7a78e 100644
--- a/packages/app-testing/src/CallDisplay/Error.js
+++ b/packages/app-testing/src/CallDisplay/Error.js
@@ -9,9 +9,12 @@ import './CallDisplay.css';
 
 import React from 'react';
 
-export default function Error (props: BaseProps): React$Node {
+export default function Error ({ className, style }: BaseProps): React$Node {
   return (
-    <div className='testing--CallDisplay-error'>
+    <div
+      className={['testing--CallDisplay-error', className].join(' ')}
+      style={style}
+    >
       ERROR: Invalid or unimplemented extrinsic function
     </div>
   );
diff --git a/packages/app-testing/src/CallSelect.js b/packages/app-testing/src/CallSelect.js
index d63ac0666d..8ae5db7d6c 100644
--- a/packages/app-testing/src/CallSelect.js
+++ b/packages/app-testing/src/CallSelect.js
@@ -14,7 +14,7 @@ import extrinsics from './extrinsics';
 
 type Props = BaseProps & {};
 
-const onChange = (event, { value }) => {
+const onChange = (event: SyntheticEvent<*>, { value }): void => {
   extrinsicName.next(value);
 };
 
diff --git a/packages/app-testing/src/InputAddress/index.js b/packages/app-testing/src/InputAddress/index.js
index 9b541dd17b..7f991d2d6d 100644
--- a/packages/app-testing/src/InputAddress/index.js
+++ b/packages/app-testing/src/InputAddress/index.js
@@ -32,7 +32,7 @@ const testOptions = keyring.getPairs().map((pair) => {
 });
 
 export default function InputAddress (props: Props): React$Node {
-  const onChange = (event, { value }) => {
+  const onChange = (event: SyntheticEvent<*>, { value }): void => {
     if (props.subject) {
       props.subject.next(value);
     }
diff --git a/packages/app-testing/src/Nonce.js b/packages/app-testing/src/Nonce.js
index 44bdc95af0..470d6d1eaa 100644
--- a/packages/app-testing/src/Nonce.js
+++ b/packages/app-testing/src/Nonce.js
@@ -16,7 +16,7 @@ import { senderAddr, senderIndex } from './subjects';
 type Props = BaseProps & {};
 
 const SenderNonce = withObservableParams(RxNonce, senderAddr);
-const onNonceChange = (value) => {
+const onNonceChange = (value): void => {
   // flowlint-next-line sketchy-null-mixed:off
   if (!value || value.length !== 8) {
     return;
diff --git a/packages/app-testing/src/Staking/Transfer/Amount.js b/packages/app-testing/src/Staking/Transfer/Amount.js
index e6965048b8..e92be864d1 100644
--- a/packages/app-testing/src/Staking/Transfer/Amount.js
+++ b/packages/app-testing/src/Staking/Transfer/Amount.js
@@ -14,7 +14,7 @@ import { amount } from './subjects';
 
 type Props = BaseProps & {};
 
-const onChange = (event, { value }) => {
+const onChange = (event: SyntheticEvent<*>, { value }): void => {
   amount.next(
     new BN(value || 0)
   );