Fix devision by zero PHP notice

src/UseCases/ValidateMembershipFee/ValidateFeeResult.php

@@ -7,6 +7,7 @@
 class ValidateFeeResult {
 
 	public const ERROR_TOO_LOW = 'error-too-low';
+	public const ERROR_INTERVAL_INVALID = 'error-interval-invalid';
 
 	private $errorCode;
 
@@ -18,6 +19,10 @@ public static function newTooLowResponse(): self {
 		return self::newErrorResponse( self::ERROR_TOO_LOW );
 	}
 
+	public static function newIntervalInvalidResponse(): self {
+		return self::newErrorResponse( self::ERROR_INTERVAL_INVALID );
+	}
+
 	private static function newErrorResponse( string $errorCode ): self {
 		$result = new self();
 		$result->errorCode = $errorCode;

src/UseCases/ValidateMembershipFee/ValidateMembershipFeeUseCase.php

@@ -28,6 +28,10 @@ public function validate( ValidateFeeRequest $request ): ValidateFeeResult {
 		$this->paymentIntervalInMonths = $request->getPaymentIntervalInMonths();
 		$this->applicantType = $request->getApplicantType();
 
+		if ( $this->paymentIntervalInMonths < 1 ) {
+			return ValidateFeeResult::newIntervalInvalidResponse();
+		}
+
 		if ( $this->getYearlyPaymentAmount() < $this->getYearlyPaymentRequirement() ) {
 			return ValidateFeeResult::newTooLowResponse();
 		}

tests/Unit/UseCases/ValidateMembershipFee/ValidateMembershipFeeUseCaseTest.php

@@ -107,4 +107,30 @@ public function testGivenInvalidCompanyAmount_validationFails(): void {
 		);
 	}
 
+	public function testGivenInvalidInterval_zero_validationFails() {
+		$useCase = $this->newUseCase();
+
+		$response = $useCase->validate(
+			ValidateFeeRequest::newInstance()
+				->withFee( Euro::newFromString( '12.34' ) )
+				->withApplicantType( ValidateFeeRequest::PERSON_APPLICANT )
+				->withInterval( 0 )
+		);
+
+		$this->assertSame( ValidateFeeResult::ERROR_INTERVAL_INVALID, $response->getErrorCode() );
+	}
+
+	public function testGivenInvalidInterval_negative_validationFails() {
+		$useCase = $this->newUseCase();
+
+		$response = $useCase->validate(
+			ValidateFeeRequest::newInstance()
+				->withFee( Euro::newFromString( '12.34' ) )
+				->withApplicantType( ValidateFeeRequest::PERSON_APPLICANT )
+				->withInterval( -1 )
+		);
+
+		$this->assertSame( ValidateFeeResult::ERROR_INTERVAL_INVALID, $response->getErrorCode() );
+	}
+
 }