dotnet/aspnetcore · error · IllegalArgumentException

The HubConnection url must be a valid url.

Error message

The HubConnection url must be a valid url.

What it means

An IllegalArgumentException thrown by setBaseUrl when the provided url is null or empty. setBaseUrl is used to repoint an existing HubConnection at a different endpoint (commonly after a redirect or for reconnection), and it guards input validity before mutating baseUrl.

Source

Thrown at src/SignalR/clients/java/signalr/core/src/main/java/com/microsoft/signalr/HubConnection.java:215

     *
     * @return HubConnection state enum.
     */
    public HubConnectionState getConnectionState() {
        return this.state.getHubConnectionState();
    }

    // For testing only
    String getBaseUrl() {
        return this.baseUrl;
    }

    /**
     * Sets a new url for the HubConnection.
     * @param url The url to connect to.
     */
    public void setBaseUrl(String url) {
        if (url == null || url.isEmpty()) {
            throw new IllegalArgumentException("The HubConnection url must be a valid url.");
        }

        if (this.state.getHubConnectionState() != HubConnectionState.DISCONNECTED) {
            throw new IllegalStateException("The HubConnection must be in the disconnected state to change the url.");
        }

        this.baseUrl = url;
    }

    /**
     * Starts a connection to the server.
     *
     * @return A Completable that completes when the connection has been established.
     */
    public Completable start() {
        CompletableSubject localStart = CompletableSubject.create();

        this.state.lock.lock();

View on GitHub (pinned to 294cab2f9b)

Solutions

1. Validate the new URL is non-null and non-empty before calling setBaseUrl, and log/skip if invalid.
2. Fix the upstream source of the URL (redirect response parsing, config provider) so it yields a real value.
3. Guard against null at the call site and fall back to the existing baseUrl.

Example fix

// before
connection.setBaseUrl(redirectResponse.getNewUrl()); // null if field absent

// after
String newUrl = redirectResponse.getNewUrl();
if (newUrl != null && !newUrl.isEmpty()) {
    connection.setBaseUrl(newUrl);
}

String newUrl = computeRedirectUrl();
if (newUrl == null || newUrl.isEmpty()) {
    logger.warn("Ignoring empty redirect URL; keeping current base URL");
} else {
    connection.setBaseUrl(newUrl);
}

try {
    connection.setBaseUrl(newUrl);
} catch (IllegalArgumentException e) {
    if (e.getMessage().contains("valid url")) {
        logger.error("Refusing to set empty base URL", e);
    } else { throw e; }
}

When it happens

Trigger: Calling hubConnection.setBaseUrl(null) or setBaseUrl("") at runtime, often when feeding a URL sourced from a dynamic config or a failed redirect parse that resolves to null/empty.

Common situations: A redirect URL or load-balancer-injected endpoint resolves to null because the response field was absent. A configuration reload yields a blank value. Code that conditionally builds the new URL and passes it without a null check.

Related errors

• A valid url is required.
• A valid url is required.
• The HubConnection url must be a valid url.
• The 'HubConnectionBuilder.withUrl' method must be called bef
• '%s' already has a value returning handler. Multiple return

AI-assisted analysis of dotnet/aspnetcore@294cab2f9b (2026-08-06). Data as JSON: /api/errors/15ba8ddc5499e4d0. Report an issue: GitHub.