BoundaryML/baml · error

nil collector found in collectors

Error message

nil collector found in collectors

What it means

When encoding the Collectors list for a function call, each collector must be a valid raw object handle. A nil element in the Collectors slice cannot be encoded as a BamlObjectHandle, so this explicit error is returned before crossing the FFI boundary.

Solutions

1. Filter out nil entries before assigning args.Collectors
2. Use collectory.NewCollector (the constructor) for each collector rather than zero-value structs
3. Check that the collector creation call returned no error and a non-nil value
4. Only pass args.Collectors when len > 0 and all entries are non-nil

Example fix

// before
collectors := make([]Collector, 2)
collectors[0] = c
args.Collectors = collectors // collectors[1] is nil
// after
collectors := []Collector{c}
args.Collectors = collectors

func hasNilCollector(cs []Collector) bool {
    for _, c := range cs { if c == nil { return true } }
    return false
}

if hasNilCollector(collectors) {
    return errors.New("collectors contains a nil entry")
}
// then proceed with args.Collectors = collectors

When it happens

Trigger: Passing a Collectors slice to BamlFunctionArguments that contains a nil entry — e.g. building the slice with make([]Collector, n) and only partially filling it, or appending a zero-valued collector.

Common situations: Preallocating a collectors slice with make(..., 2) but registering only one; a collector variable that failed to initialize earlier (err ignored); copying collectors between calls with a nil placeholder.

Related errors

• encoding collector
• decodeLiteralValue: valueLiteral is nil
• destructor returned unexpected result
• encoding client registry
• encoding env vars

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/eeb08313df0f5e49. Report an issue: GitHub.

Appendix: source

Thrown at engine/language_client_go/pkg/rawobjects_function_args.go:55

		clientRegistry, err = encodeClientRegistry(args.ClientRegistry)
		if err != nil {
			return nil, fmt.Errorf("encoding client registry: %w", err)
		}
	}

	var env []*cffi.HostEnvVar
	if args.Env != nil {
		env, err = serde.EncodeEnvVar(args.Env)
		if err != nil {
			return nil, fmt.Errorf("encoding env vars: %w", err)
		}
	}

	var collectors []*cffi.BamlObjectHandle
	if args.Collectors != nil {
		for _, collector := range args.Collectors {
			if collector == nil {
				return nil, fmt.Errorf("nil collector found in collectors")
			}
			encodedCollector := raw_objects.EncodeRawObject(collector)
			if err != nil {
				return nil, fmt.Errorf("encoding collector: %w", err)
			}
			collectors = append(collectors, encodedCollector)
		}
	}

	var typeBuilder *cffi.BamlObjectHandle
	if args.TypeBuilder != nil {
		encodedTypeBuilder := raw_objects.EncodeRawObject(args.TypeBuilder)
		if err != nil {
			return nil, fmt.Errorf("encoding type builder: %w", err)
		}
		typeBuilder = encodedTypeBuilder
	}

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