apache/beam · error

Invalid Coder

Error message

Invalid Coder

What it means

Coder.Type() returns the FullType of elements a Beam Go coder can encode/decode, but panics with 'Invalid Coder' if the Coder is the zero value (its internal coder pointer is nil) — i.e. IsValid() is false. The library treats calling Type() on an invalid coder as a programming error.

Source

Thrown at sdks/go/pkg/beam/coder.go:80

// Coder defines how to encode and decode values of type 'A' into byte streams.
// Coders are attached to PCollections of the same type. For PCollections
// consumed by GBK, the attached coders are required to be deterministic.
type Coder struct {
	coder *coder.Coder
}

// IsValid returns true iff the Coder is valid. Any use of an invalid Coder
// will result in a panic.
func (c Coder) IsValid() bool {
	return c.coder != nil
}

// Type returns the full type 'A' of elements the coder can encode and decode.
// 'A' must be a concrete full type, such as int or KV<int,string>.
func (c Coder) Type() FullType {
	if !c.IsValid() {
		panic("Invalid Coder")
	}
	return c.coder.T
}

func (c Coder) String() string {
	if c.coder == nil {
		return "$"
	}
	return c.coder.String()
}

// IsDeterministic reports whether this coder produces a byte-deterministic
// encoding: encoding two equal values always yields identical byte
// sequences.
//
// Determinism is required for any coder used as a state key in a stateful
// DoFn or as the key component of a KV consumed by GroupByKey /
// GroupIntoBatches. A non-deterministic key coder would silently corrupt

View on GitHub (pinned to 12126d8942)

Solutions

1. Call c.IsValid() before calling Type() and handle the invalid case explicitly
2. Ensure the Coder was constructed via beam.NewCoder (checking its returned error upstream)
3. Trace where the zero-value Coder originated — usually an unchecked error earlier in construction

Example fix

// before
fmt.Println(coder.Type())
// after
if coder.IsValid() {
	fmt.Println(coder.Type())
} else {
	return errors.New("coder was never initialized")
}

if !c.IsValid() {
	return errors.New("coder not initialized")
}

func (c Coder) IsValid() bool { return c.coder != nil }

func safeType(c beam.Coder) (t typex.FullType, err error) {
	defer func() {
		if r := recover(); r != nil {
			err = fmt.Errorf("invalid coder: %v", r)
		}
	}()
	return c.Type(), nil
}

When it happens

Trigger: Declaring a beam.Coder variable without initializing it (var c beam.Coder; c.Type()) or receiving a zero-value Coder from a struct field/map lookup that was never populated, then calling Type().

Common situations: Storing Coders in maps or structs with missing initialization; error-swallowing constructors that returned an empty Coder; reflection-driven code paths building coders dynamically.

Related errors

• err
• inferCoder: unknown container kind %v
• Unexpected composite type: %v
• Unexpected type: %v
• coder must not be nil

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/caec674c9c7c0a2f. Report an issue: GitHub.