GitHub Actions "startsWith ... wrong argument type" in CI

By Kaveh Alemi·Latchkey

startsWith and endsWith compare two strings. Passing a null context value, an object, or an array makes the expression evaluate to an unexpected result or fail, because the argument is not a string.

What this error means

An if using startsWith(...) never matches as expected, or the run reports a type problem, when one argument is a context that resolves to null or to a non-string.

Error: Unexpected type of value '' for parameter 'searchString' in startsWith()

Common causes

A context resolves to null

A field like github.event.pull_request.title can be null on non-PR events, so startsWith gets a non-string first argument.

An object or array passed instead of a string

Handing github.event or a JSON array to startsWith violates its string-only signature.

How to fix it

Coerce to a string and guard for null

1. Wrap the value so it is always a string, e.g. format('{0}', x).
2. Add an event guard so the field exists before comparing.
3. Re-run the workflow.

if: ${{ startsWith(format('{0}', github.ref), 'refs/tags/') }}

Use the right field for the event

Reference a field that is always a string for the triggering event, such as github.ref instead of a PR-only field.

How to prevent it

• Assume PR-only fields are null on push events.
• Use format() to coerce values before string functions.
• Test if conditions across the events that can trigger them.

Frequently asked questions

Related guides

References

• GitHub Actions documentation
• Workflow syntax for GitHub Actions