element : expected boolean element to be bool, but got %T

Error message

element %d: expected boolean element to be bool, but got %T

What it means

mergeBooleanProps reduces grouped boolean values during groupBy (majority vote: returns true if countTrue >= countFalse pattern of counting). Every element must be a Go bool; anything else aborts with the index and actual type.

Solutions

Verify the property dataType is boolean and values were imported as real JSON booleans
Fix clients writing "true" strings or 1/0 into boolean properties
Re-import or update offending objects with proper boolean values
Check the error's element index and property name (from the wrapping 'prop' error) to locate the bad data

Example fix

// before (bad import)
{"hasStock": "true"}
// after
{"hasStock": true}

Defensive patterns

Strategy: validation

Validate before calling

// Validate booleans before import
for (const obj of objects) {
  if (obj.inStock !== undefined && typeof obj.inStock !== 'boolean') {
    throw new Error('inStock must be a real boolean, got ' + typeof obj.inStock)
  }
}

Prevention

Send true/false, not "true"/"false" or 1/0, for boolean properties
Add client-side schema validation before batch imports
Re-check legacy data written by old clients with string booleans
Use the error's element index to locate the corrupt object when debugging

When it happens

Trigger: A groupBy query over a boolean property where an extracted element is not bool — e.g. the value was written as the string "true"/"false" or 0/1 and decoded as string/number.

Common situations: Clients sending booleans as strings or numbers on import, causing stored/extracted values to deserialize into non-bool types during grouped merging.

Related errors

AI-assisted analysis of weaviate/weaviate@75aa4b6d11 (2026-09-04). Data as JSON: /api/errors/57c506ec635ce536. Report an issue: GitHub.

Appendix: source

Thrown at usecases/traverser/grouper/merge_group.go:227

	for i, elem := range in {
		asFloat, ok := elem.(float64)
		if !ok {
			return 0, fmt.Errorf("element %d: expected numerical element to be float64, but got %T", i, elem)
		}

		sum += asFloat
	}

	return sum / float64(len(in)), nil
}

func mergeBooleanProps(in []interface{}) (bool, error) {
	var countTrue uint
	var countFalse uint
	for i, elem := range in {
		asBool, ok := elem.(bool)
		if !ok {
			return false, fmt.Errorf("element %d: expected boolean element to be bool, but got %T", i, elem)
		}

		if asBool {
			countTrue++
		} else {
			countFalse++
		}
	}

	return countTrue >= countFalse, nil
}

func mergeGeoProps(in []interface{}) (*models.GeoCoordinates, error) {
	var sumLat float32
	var sumLon float32

	for i, elem := range in {
		asGeo, ok := elem.(*models.GeoCoordinates)

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