vectordotdev/vector · error

argument must be a string

Error message

argument must be a string

What it means

get_vector_metric's helper starts with key.as_str().expect("argument must be a string") (lib/vector-vrl/metrics/src/get_vector_metric.rs). Its key parameter is defined as Parameter::required("key", kind::BYTES, ...), so VRL's compiler enforces bytes at the call site; the expect is a runtime assertion of that compile-time contract and signals an internal invariant violation if it ever fires.

Source

Thrown at lib/vector-vrl/metrics/src/get_vector_metric.rs:15

use std::{collections::BTreeMap, sync::LazyLock};

use vector_vrl_category::Category;
use vrl::prelude::{expression::Expr, *};

use crate::common::{
    metric_into_vrl, metrics_vrl_typedef, resolve_tags, validate_tags, Error, MetricsStorage,
};

fn get_metric(
    metrics_storage: &MetricsStorage,
    key: Value,
    tags: BTreeMap<String, String>,
) -> Result<Value, ExpressionError> {
    let key_str = key.as_str().expect("argument must be a string");
    let value = match metrics_storage.get_metric(&key_str, tags) {
        Some(value) => metric_into_vrl(&value),
        None => Value::Null,
    };
    Ok(value)
}

static DEFAULT_TAGS: LazyLock<Value> = LazyLock::new(|| Value::Object(BTreeMap::new()));

static PARAMETERS: LazyLock<Vec<Parameter>> = LazyLock::new(|| {
    vec![
        Parameter::required("key", kind::BYTES, "The metric name to search."),
        Parameter::optional(
            "tags",
            kind::OBJECT,
            "Tags to filter the results on. Values in this object support wildcards ('*') to match on parts of the tag value.",
        )
        .default(&DEFAULT_TAGS),

View on GitHub (pinned to 3708c39b12)

Solutions

1. Upgrade Vector/vector-vrl crates in lockstep
2. Pass statically-typed strings: get_vector_metric("buffer_byte_size", tags = {...})
3. Use the compiled Program API for embedding
4. Report reproducible stock-build panics to vectordot/vector

Example fix

# before (vrl)
get_vector_metric(key)   # key: any

# after (vrl)
get_vector_metric(to_string!(key))

# Literal or coerced metric key
get_vector_metric(to_string!(key))

fn is_bytes(v: &vrl::value::Value) -> bool {
    matches!(v, vrl::value::Value::Bytes(_))
}

When it happens

Trigger: Executing get_vector_metric with a runtime key Value that is not bytes/string — prevented by compilation in normal flows (get_vector_metric(true) fails to compile); occurs only with VRL compiler bugs, version-mismatched vector-vrl crates, or direct helper use outside the compile pipeline.

Common situations: Embedding VRL without the type-checking pass; dependency skew between vector-vrl-metrics and the compiler; compiler regression testing.

Related errors

• argument must be a string
• argument must be a string
• argument must be a string
• argument must be a string
• key must be a string

AI-assisted analysis of vectordotdev/vector@3708c39b12 (2026-08-20). Data as JSON: /api/errors/6a69c65f13e8012e. Report an issue: GitHub.