dotnet/aspnetcore · error · Error

The '${name}' argument is required.

Error message

The '${name}' argument is required.

What it means

Arg.isRequired is a guard used across public APIs (connection builders, transport.connect, hub methods) that throws when a mandatory argument is null or undefined. It is the standard precondition check for required parameters.

Source

Thrown at src/SignalR/clients/ts/signalr/src/Utils.ts:20

// The .NET Foundation licenses this file to you under the MIT license.

import { HttpClient } from "./HttpClient";
import { ILogger, LogLevel } from "./ILogger";
import { NullLogger } from "./Loggers";
import { IStreamSubscriber, ISubscription } from "./Stream";
import { Subject } from "./Subject";
import { IHttpConnectionOptions } from "./IHttpConnectionOptions";
import { VERSION } from "./pkg-version";

// Version token that will be replaced by the prepack command
/** The version of the SignalR client. */

export { VERSION };
/** @private */
export class Arg {
    public static isRequired(val: any, name: string): void {
        if (val === null || val === undefined) {
            throw new Error(`The '${name}' argument is required.`);
        }
    }
    public static isNotEmpty(val: string, name: string): void {
        if (!val || val.match(/^\s*$/)) {
            throw new Error(`The '${name}' argument should not be empty.`);
        }
    }

    public static isIn(val: any, values: any, name: string): void {
        // TypeScript enums have keys for **both** the name and the value of each enum member on the type itself.
        if (!(val in values)) {
            throw new Error(`Unknown ${name} value: ${val}.`);
        }
    }
}

/** @private */
export class Platform {

View on GitHub (pinned to 294cab2f9b)

Solutions

1. Pass a concrete value for the argument named in the message.
2. Initialize variables before use and provide defaults at the call site.
3. Add your own Arg.isRequired or null check earlier to surface the missing value with more context.
4. Review the stack trace to find which API call omitted the argument.

Example fix

// before
const url = config.url; // undefined
const conn = new HubConnection(url); // throws

// after
const url = config.url;
if (url == null) throw new Error('config.url is required');
const conn = new HubConnection(url);

function requireArg<T>(val: T | null | undefined, name: string): T {
  if (val === null || val === undefined) {
    throw new Error(`Missing required argument '${name}'`);
  }
  return val;
}

function isPresent<T>(v: T | null | undefined): v is T {
  return v !== null && v !== undefined;
}

When it happens

Trigger: Calling an internal/public API with null or undefined for a required parameter, e.g. withUrl(null), transport.connect(undefined, format), or Arg.isRequired(myVar, 'myVar') where myVar is undefined.

Common situations: Passing an uninitialized variable, a config object whose field is missing, or destructuring defaults that resolve to undefined. Pure programmer error.

Related errors

• The '${name}' argument should not be empty.
• Unknown ${name} value: ${val}.
• Invalid input for JSON hub protocol. Expected a string.
• Invalid payload.
• Invalid payload for StreamItem message.

AI-assisted analysis of dotnet/aspnetcore@294cab2f9b (2026-08-06). Data as JSON: /api/errors/5d06e3e1210ea206. Report an issue: GitHub.