go-playground/validator · error

Bad field type %s:%s

Error message

Bad field type %s:%s

What it means

Panics inside isUnique's default branch when the sibling field named by the unique tag parameter exists but its reflect.Kind differs from the field being validated. unique performs a direct value comparison between the two fields, which is only meaningful for like kinds; the panic message prints both types (field type : unique field type) to expose the mismatch.

Source

Thrown at baked_in.go:484

			}

			if _, ok := seen[key]; ok {
				return false
			}
			seen[key] = struct{}{}
		}

		return true

	default:
		if parent := fl.Parent(); parent.Kind() == reflect.Struct {
			uniqueField := parent.FieldByName(param)
			if !uniqueField.IsValid() {
				panic(fmt.Sprintf("Bad field name provided %s", param))
			}

			if uniqueField.Kind() != field.Kind() {
				panic(fmt.Sprintf("Bad field type %s:%s", field.Type(), uniqueField.Type()))
			}

			return getValue(field) != getValue(uniqueField)
		}

		panic(fmt.Sprintf("Bad field type %s", field.Type()))
	}
}

// isMAC is the validation function for validating if the field's value is a valid MAC address.
func isMAC(fl FieldLevel) bool {
	_, err := net.ParseMAC(fl.Field().String())

	return err == nil
}

// isCIDRv4 is the validation function for validating if the field's value is a valid v4 CIDR address.
func isCIDRv4(fl FieldLevel) bool {

View on GitHub (pinned to facf128d2e)

Solutions

1. Make both fields the same type so the comparison is valid
2. Convert one field's type to match the other (e.g. int32 vs int64, or one side a pointer)
3. Write a custom validator if cross-type uniqueness comparison is genuinely needed

When it happens

Trigger: Thrown at baked_in.go:484 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of go-playground/validator@facf128d2e (2026-09-02). Data as JSON: /api/errors/0388344df2e86a16. Report an issue: GitHub.