unexpected number of parameters in method

Error message

unexpected number of parameters in method %v. got: %v, want %v. Check that the signature conforms to the expected signature for %v, and that elements in SDF method parameters match elements in %v.

What it means

In a stateful SDF, WatermarkEstimatorState must take exactly one parameter — the sdf.WatermarkEstimator whose state is being read. Beam validates len(method.Param) == 1 and throws this error when the method has zero parameters or extra ones (e.g. context or restriction), because the runner calls it with just the estimator.

Solutions

Change WatermarkEstimatorState to take exactly one parameter of type sdf.WatermarkEstimator.
Move any needed context or restriction access into the estimator itself (store it when created) or into other methods.
Verify against the documented stateful SDF signature table.
Check the method name — Beam matches by exact name, so a differently named method won't satisfy this check.

Example fix

// before
func (f *fn) WatermarkEstimatorState(ctx context.Context, we sdf.WatermarkEstimator) myState { ... }
// after
func (f *fn) WatermarkEstimatorState(we sdf.WatermarkEstimator) myState {
    return we.(*myWatermarkEstimator).state
}

Defensive patterns

Strategy: validation

Validate before calling

var _ func(sdf.WatermarkEstimator) myState = (*fn).WatermarkEstimatorState

Prevention

WatermarkEstimatorState takes only the estimator — no context, no restriction.
Store any needed context data in the estimator when it's created.
Lock the signature with a compile-time assertion.

When it happens

Trigger: Declaring WatermarkEstimatorState(ctx context.Context, we sdf.WatermarkEstimator) or WatermarkEstimatorState(rest MyRestriction, we sdf.WatermarkEstimator); omitting the estimator parameter entirely.

Common situations: Adding context out of habit from other DoFn methods; copying ProcessElement's parameter style; misunderstanding which methods receive extra SDF parameters.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/b07228cd9d45cab1. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/graph/fn.go:1312

			if len(method.Ret) != 1 {
				err := errors.Errorf("unexpected number of elements returned in method %v. got: %v, want %v",
					initialWatermarkEstimatorStateName, len(method.Ret), 1)
				return errors.SetTopLevelMsgf(err, "unexpected number of elements returned in method %v. "+
					"got: %v, want %v. Check that the signature conforms to the expected signature for %v.",
					initialWatermarkEstimatorStateName, len(method.Ret), 1, initialWatermarkEstimatorStateName)
			}
			if method.Ret[0].T != watermarkStateT {
				err := errors.Errorf("mismatched output type in method %v, return %v. got: %v, want: %v",
					createWatermarkEstimatorName, 0, method.Ret[0].T, watermarkStateT)
				return errors.SetTopLevelMsgf(err, "mismatched output type in method %v, "+
					"return value at index %v got: %v, want: %v (from method %v). "+
					"Ensure that all watermark states in an SDF are the same type.",
					createWatermarkEstimatorName, 0, method.Ret[0].T, watermarkStateT, createWatermarkEstimatorName)
			}
		case watermarkEstimatorStateName:
			if len(method.Param) != 1 {
				err := errors.Errorf("unexpected number of params in method %v. got: %v, want %v",
					watermarkEstimatorStateName, len(method.Param), 1)
				return errors.SetTopLevelMsgf(err, "unexpected number of parameters in method %v. "+
					"got: %v, want %v. Check that the signature conforms to the expected signature for %v, "+
					"and that elements in SDF method parameters match elements in %v.",
					watermarkEstimatorStateName, len(method.Param), 1, watermarkEstimatorStateName, processElementName)
			}
			if method.Param[0].T != watermarkEstimatorT {
				err := errors.Errorf("mismatched watermark state type in method %v, return %v. got: %v, want: %v",
					watermarkEstimatorStateName, 0, method.Param[0].T, watermarkEstimatorT)
				return errors.SetTopLevelMsgf(err, "mismatched watermark state type in method %v, "+
					"return value at index %v got: %v, want: %v (from method %v). "+
					"Ensure that all watermark states in an SDF are the same type.",
					watermarkEstimatorStateName, 0, method.Param[0].T, watermarkEstimatorT, watermarkEstimatorStateName)
			}
			if len(method.Ret) != 1 {
				err := errors.Errorf("unexpected number of elements returned in method %v. got: %v, want %v",
					watermarkEstimatorStateName, len(method.Ret), 1)
				return errors.SetTopLevelMsgf(err, "unexpected number of elements returned in method %v. "+

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