pulumi/pulumi · error

result field type %s does not match expected type %s

Error message

result field type %s does not match expected type %s

What it means

After unwrapping the single field, CallPackageSingle verifies the field's Go type matches the ElementType of the requested output. A mismatch yields 'result field type <actual> does not match expected type <expected>', preventing an unsafe reflective assignment.

Source

Thrown at sdk/go/pulumi/context.go:1293

			if v.IsNil() {
				return zeroType, errors.New("input cannot be a nil pointer")
			}
			// Get the element the pointer points to
			v = v.Elem()
		}

		if v.Kind() != reflect.Map {
			return zeroType, errors.New("result must be a map, but was a " + v.Kind().String())
		}

		asMap := v.Interface().(map[string]any)
		if len(asMap) != 1 {
			return zeroType, errors.New("result must have exactly one element")
		}

		result := slices.Collect(maps.Values(asMap))[0]
		if resultType := reflect.TypeOf(result); resultType != output.ElementType() {
			return zeroType, fmt.Errorf("result field type %s does not match expected type %s", resultType, output.ElementType())
		}

		return result, nil
	})

	outputState := internal.GetOutputState(intermediary.(AnyOutput))
	if outputState == nil {
		return nil, errors.New("intermediary output state is nil")
	}

	resValue := reflect.New(reflect.TypeOf(output)).Elem()
	internal.SetOutputState(resValue, outputState)
	res := resValue.Interface().(Output)

	return res, nil
}

// ReadResource reads an existing custom resource's state from the resource monitor. t is the fully qualified type

View on GitHub (pinned to 793f7b2e16)

Solutions

1. Match the output type to the actual field type (e.g. use pulumi.Float64Output for a number field).
2. Consult the provider schema for the function's return type and adjust the requested Output type.
3. If the provider changed types, pin the previous provider version or update program code.

Example fix

// before
out, err := ctx.CallPackageSingle(tok, args, pulumi.StringOutput{}, nil, "") // field is int
// after
out, err := ctx.CallPackageSingle(tok, args, pulumi.Float64Output{}, nil, "")

// confirm the return field type in the provider schema matches the Output element type you request
// schema.functions[tok].returns.properties[<field>].type == "string" for pulumi.StringOutput

func matchesElementType(result any, out pulumi.Output) bool {
    return reflect.TypeOf(result) == out.ElementType()
}

out, err := ctx.CallPackageSingle(tok, args, pulumi.StringOutput{}, nil, "")
if err != nil && strings.Contains(err.Error(), "does not match expected type") {
    return fmt.Errorf("wrong output type requested for %s; see wrapped error for actual type", tok)
}

When it happens

Trigger: Requesting e.g. pulumi.StringOutput for a provider function whose single return field is a number/bool, or vice versa; provider changed the field's type across versions.

Common situations: Copy-pasting a Single call from another function with a different return type; schema changes in a provider upgrade altering a property's type.

Related errors

• result must be a map, but was a
• applier's %s input parameter must be assignable from %v, got
• cannot convert an input of type %T to a value of type %v
• input cannot be a nil pointer
• result must have exactly one element

AI-assisted analysis of pulumi/pulumi@793f7b2e16 (2026-08-31). Data as JSON: /api/errors/3924ae51cb84ca48. Report an issue: GitHub.