apache/beam · error

input to MultiMap side input must be KV, got

Error message

input to MultiMap side input must be KV, got %v

What it means

Error returned by tryBindInbound in the graph binder when a side input of kind funcx.FnMultiMap is being bound and the PCollection type t is not KV. MultiMap side inputs expose per-key iterables, which requires the side-input collection to be a KV<K, V> type; any other full type is rejected with this message showing the offending type.

Solutions

1. Feed the MultiMap side input a KV<K,V> PCollection (apply beam.CoGBK or a KV conversion before the ParDo)
2. Use a different side-input kind (e.g. Iter/ReIter) if the collection is not KV

When it happens

Trigger: Thrown at sdks/go/pkg/beam/core/graph/bind.go:218 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/769cc3dd069819ce. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/graph/bind.go:218

				if len(trimmed) != 1 {
					return nil, kind, errors.Errorf("%v cannot bind to %v", t, args[0])
				}

				kind = Iter
				other = typex.New(trimmed[0])

			case funcx.FnReIter:
				values, _ := funcx.UnfoldReIter(args[0].T)
				trimmed := trimIllegal(values)
				if len(trimmed) != 1 {
					return nil, kind, errors.Errorf("%v cannot bind to %v", t, args[0])
				}

				kind = ReIter
				other = typex.New(trimmed[0])

			case funcx.FnMultiMap:
				return nil, kind, errors.Errorf("input to MultiMap side input must be KV, got %v", t)

			default:
				return nil, kind, errors.Errorf("unexpected param kind: %v", arg)
			}
		}
	case typex.Composite:
		switch t.Type() {
		case typex.KVType:
			if isMain {
				if args[0].Kind != funcx.FnValue {
					return nil, kind, errors.Errorf("key of %v cannot bind to %v", t, args[0])
				}
				if args[1].Kind != funcx.FnValue {
					return nil, kind, errors.Errorf("value of %v cannot bind to %v", t, args[1])
				}
				other = typex.NewKV(typex.New(args[0].T), typex.New(args[1].T))
			} else {
				switch args[0].Kind {

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