oven-sh/bun

Misleading error message "URL must not be a blank string" when URL is a non-string value which does not stringify to a fully valid URL with a hostname

Open

#21361 opened on Jul 24, 2025

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 (6 comments) (1 reaction) (0 assignees)Rust (90,348 stars) (4,486 forks)batch import
bugconfirmed buggood first issueweb:fetch

Description

What version of Bun is running?

1.2.18+0d4089ea7

What platform is your computer?

Darwin 24.3.0 arm64 arm

What steps can reproduce the bug?

Any of the following (and more):

await fetch(undefined)
await fetch(123)
await fetch({toString() {return `/some_path`}})

What is the expected behavior?

Something more precise, like:

fetch() URL must have a hostname

What do you see instead?

fetch() URL must not be a blank string

Additional information

Link to offending line in Bun sources: https://github.com/oven-sh/bun/blob/bd232189b4c39ac3c8ccef9cf0932407d65c527a/src/bun.js/webcore/fetch.zig#L1448

Contributor guide

Tech stack
javascript
Domain
backenddeveloper experience
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Understanding of Zig programmingFamiliarity with Bun's build systemBasic knowledge of JavaScript fetch API
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
75
Research direction
The issue is clearly described with the offending line at src/bun.js/webcore/fetch.zig:1448. The fix involves changing the error message string to be more specific about the URL missing a hostname. The contributor should examine the URL validation logic in that file to understand how the error is generated and ensure the new message is accurate. No linked PRs or additional comments provide further guidance.