diff --git a/CHANGELOG.md b/CHANGELOG.md
index 904fc8f..ce30d1e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -21,6 +21,7 @@
 - Prevent a machine from changing its owner to itself (closes [#373](https://github.com/tale/headplane/issues/373)).
 - Added an `/admin/api/info` route that can expose sensitive information if `server.info_secret` is set in the configuration (closes [#324](https://github.com/tale/headplane/issues/324)).
 - Correctly apply Gravatar profile pictures on the user page if applicable (closes [#405](https://github.com/tale/headplane/issues/405)).
+- Machine key registration no longer works if the key isn't 24 characters long (closes [#415](https://github.com/tale/headplane/issues/415)).
 ---
 
 # 0.6.1 (October 12, 2025)
diff --git a/app/components/Dialog.tsx b/app/components/Dialog.tsx
index d30d25c..9eb4e78 100644
--- a/app/components/Dialog.tsx
+++ b/app/components/Dialog.tsx
@@ -107,6 +107,11 @@ function Panel(props: DialogPanelProps) {
 	return (
 		<Form
 			{...dialogProps}
+			className={cn(
+				'outline-hidden rounded-3xl w-full max-w-lg',
+				'bg-white dark:bg-headplane-900',
+			)}
+			method={method ?? 'POST'}
 			onSubmit={(event) => {
 				if (onSubmit) {
 					onSubmit(event);
@@ -114,12 +119,7 @@ function Panel(props: DialogPanelProps) {
 
 				close?.();
 			}}
-			method={method ?? 'POST'}
 			ref={ref}
-			className={cn(
-				'outline-hidden rounded-3xl w-full max-w-lg',
-				'bg-white dark:bg-headplane-900',
-			)}
 		>
 			<Card className="w-full max-w-lg" variant="flat">
 				{children}
@@ -130,9 +130,9 @@ function Panel(props: DialogPanelProps) {
 						<>
 							<Button onPress={close}>Cancel</Button>
 							<Button
+								isDisabled={isDisabled}
 								type="submit"
 								variant={variant === 'destructive' ? 'danger' : 'heavy'}
-								isDisabled={isDisabled}
 							>
 								Confirm
 							</Button>
diff --git a/app/components/Input.tsx b/app/components/Input.tsx
index a402263..efa1252 100644
--- a/app/components/Input.tsx
+++ b/app/components/Input.tsx
@@ -8,11 +8,18 @@ export interface InputProps extends AriaTextFieldProps<HTMLInputElement> {
 	labelHidden?: boolean;
 	isRequired?: boolean;
 	className?: string;
+	isInvalid?: boolean;
+	errorMessage?: string;
 }
 
-// TODO: Custom isInvalid logic for custom error messages
 export default function Input(props: InputProps) {
-	const { label, labelHidden, className } = props;
+	const {
+		label,
+		labelHidden,
+		className,
+		isInvalid: customIsInvalid,
+		errorMessage,
+	} = props;
 	const ref = useRef<HTMLInputElement | null>(null);
 	const id = useId(props.id);
 
@@ -21,7 +28,7 @@ export default function Input(props: InputProps) {
 		inputProps,
 		descriptionProps,
 		errorMessageProps,
-		isInvalid,
+		isInvalid: ariaIsInvalid,
 		validationErrors,
 	} = useTextField(
 		{
@@ -32,16 +39,18 @@ export default function Input(props: InputProps) {
 		ref,
 	);
 
+	const isInvalid = customIsInvalid ?? ariaIsInvalid;
+
 	return (
-		<div className="flex flex-col w-full" aria-label={label}>
+		<div className="flex flex-col w-full">
 			<label
 				{...labelProps}
-				htmlFor={id}
 				className={cn(
 					'text-xs font-medium px-3 mb-0.5',
 					'text-headplane-700 dark:text-headplane-100',
 					labelHidden && 'sr-only',
 				)}
+				htmlFor={id}
 			>
 				{label}
 				{props.isRequired && (
@@ -50,8 +59,6 @@ export default function Input(props: InputProps) {
 			</label>
 			<input
 				{...inputProps}
-				required={props.isRequired}
-				ref={ref}
 				className={cn(
 					'rounded-xl px-3 py-2',
 					'focus:outline-hidden focus:ring-3',
@@ -59,6 +66,8 @@ export default function Input(props: InputProps) {
 					'border border-headplane-100 dark:border-headplane-800',
 					className,
 				)}
+				ref={ref}
+				required={props.isRequired}
 			/>
 			{props.description && (
 				<div
@@ -76,7 +85,7 @@ export default function Input(props: InputProps) {
 					{...errorMessageProps}
 					className={cn('text-xs px-3 mt-1', 'text-red-500 dark:text-red-400')}
 				>
-					{validationErrors.join(' ')}
+					{errorMessage ?? validationErrors.join(' ')}
 				</div>
 			) : null}
 		</div>
diff --git a/app/routes/machines/dialogs/new.tsx b/app/routes/machines/dialogs/new.tsx
index 53e74a9..bb9bddb 100644
--- a/app/routes/machines/dialogs/new.tsx
+++ b/app/routes/machines/dialogs/new.tsx
@@ -20,10 +20,12 @@ export default function NewMachine(data: NewMachineProps) {
 	const [mkey, setMkey] = useState('');
 	const navigate = useNavigate();
 
+	const isMkeyInvalid = mkey.length > 0 && mkey.length !== 24;
+
 	return (
 		<>
 			<Dialog isOpen={pushDialog} onOpenChange={setPushDialog}>
-				<Dialog.Panel isDisabled={mkey.length < 1}>
+				<Dialog.Panel isDisabled={mkey.length !== 24}>
 					<Dialog.Title>Register Machine Key</Dialog.Title>
 					<Dialog.Text className="mb-4">
 						The machine key is given when you run{' '}
@@ -32,6 +34,8 @@ export default function NewMachine(data: NewMachineProps) {
 					</Dialog.Text>
 					<input name="action_id" type="hidden" value="register" />
 					<Input
+						errorMessage="Machine key must be exactly 24 characters"
+						isInvalid={isMkeyInvalid}
 						isRequired
 						label="Machine Key"
 						name="register_key"