diff --git a/src/wp-includes/cron.php b/src/wp-includes/cron.php
index 7bbb0036f1c02..97336c3aaf566 100644
--- a/src/wp-includes/cron.php
+++ b/src/wp-includes/cron.php
@@ -316,6 +316,17 @@ function wp_schedule_event( $timestamp, $recurrence, $hook, $args = array(), $wp
 		return false;
 	}
 
+	if ( ! is_numeric( $event->interval ) || $event->interval <= 0 ) {
+		if ( $wp_error ) {
+			return new WP_Error(
+				'invalid_interval',
+				__( 'Event schedule has invalid interval.' )
+			);
+		}
+
+		return false;
+	}
+
 	$key = md5( serialize( $event->args ) );
 
 	$crons = _get_cron_array();
@@ -444,12 +455,19 @@ function wp_reschedule_event( $timestamp, $recurrence, $hook, $args = array(), $
 	}
 
 	// Now we assume something is wrong and fail to schedule.
-	if ( 0 === $interval ) {
+	if ( ! is_numeric( $interval ) || $interval <= 0 ) {
 		if ( $wp_error ) {
-			return new WP_Error(
-				'invalid_schedule',
-				__( 'Event schedule does not exist.' )
-			);
+			if ( ! isset( $schedules[ $recurrence ] ) ) {
+				return new WP_Error(
+					'invalid_schedule',
+					__( 'Event schedule does not exist.' )
+				);
+			} else {
+				return new WP_Error(
+					'invalid_interval',
+					__( 'Event schedule has invalid interval.' )
+				);
+			}
 		}
 
 		return false;
diff --git a/tests/phpunit/tests/cron.php b/tests/phpunit/tests/cron.php
index 7abaf5c92095e..b4536c499fa82 100644
--- a/tests/phpunit/tests/cron.php
+++ b/tests/phpunit/tests/cron.php
@@ -847,6 +847,7 @@ public function test_invalid_timestamp_for_event_returns_error() {
 
 	/**
 	 * @ticket 49961
+	 * @ticket 64404
 	 *
 	 * @covers ::wp_schedule_event
 	 * @covers ::wp_reschedule_event
@@ -862,6 +863,99 @@ public function test_invalid_recurrence_for_event_returns_error() {
 		$this->assertSame( 'invalid_schedule', $rescheduled_event->get_error_code() );
 	}
 
+	/**
+	 * @ticket 64404
+	 *
+	 * @covers ::wp_schedule_event
+	 * @covers ::wp_reschedule_event
+	 */
+	public function test_invalid_schedule_interval_returns_error() {
+		add_filter(
+			'cron_schedules',
+			static function ( $schedules ) {
+				$schedules['backwards_in_time'] = array(
+					'interval' => -3600,
+					'display'  => 'Back to the future!',
+				);
+				return $schedules;
+			}
+		);
+
+		$event = wp_schedule_event( time(), 'backwards_in_time', 'hook', array(), true );
+		$this->assertWPError( $event );
+		$this->assertSame( 'invalid_interval', $event->get_error_code() );
+
+		$rescheduled_event = wp_reschedule_event( time(), 'backwards_in_time', 'hook', array(), true );
+		$this->assertWPError( $rescheduled_event );
+		$this->assertSame( 'invalid_interval', $rescheduled_event->get_error_code() );
+	}
+
+	/**
+	 * @ticket 64404
+	 *
+	 * @covers ::wp_schedule_event
+	 * @covers ::wp_reschedule_event
+	 */
+	public function test_zero_schedule_interval_returns_error() {
+		add_filter(
+			'cron_schedules',
+			static function ( $schedules ) {
+				$schedules['zero_interval'] = array(
+					'interval' => 0,
+					'display'  => 'Zero interval',
+				);
+				return $schedules;
+			}
+		);
+
+		$event = wp_schedule_event( time(), 'zero_interval', 'hook', array(), true );
+		$this->assertWPError( $event );
+		$this->assertSame( 'invalid_interval', $event->get_error_code() );
+
+		$rescheduled_event = wp_reschedule_event( time(), 'zero_interval', 'hook', array(), true );
+		$this->assertWPError( $rescheduled_event );
+		$this->assertSame( 'invalid_interval', $rescheduled_event->get_error_code() );
+	}
+
+	/**
+	 * @ticket 64404
+	 *
+	 * @covers ::wp_schedule_event
+	 * @covers ::wp_reschedule_event
+	 */
+	public function test_non_numeric_schedule_interval_returns_error() {
+		add_filter(
+			'cron_schedules',
+			static function ( $schedules ) {
+				$schedules['non_numeric_interval_string'] = array(
+					'interval' => 'invalid',
+					'display'  => 'Non numeric interval (string)',
+				);
+				$schedules['non_numeric_interval_null']   = array(
+					'interval' => null,
+					'display'  => 'Non numeric interval (null)',
+				);
+				return $schedules;
+			}
+		);
+
+		$string_event = wp_schedule_event( time(), 'non_numeric_interval_string', 'hook', array(), true );
+		$this->assertWPError( $string_event );
+		$this->assertSame( 'invalid_interval', $string_event->get_error_code() );
+
+		$string_rescheduled_event = wp_reschedule_event( time(), 'non_numeric_interval_string', 'hook', array(), true );
+		$this->assertWPError( $string_rescheduled_event );
+		$this->assertSame( 'invalid_interval', $string_rescheduled_event->get_error_code() );
+
+		$null_event = wp_schedule_event( time(), 'non_numeric_interval_null', 'hook', array(), true );
+		$this->assertWPError( $null_event );
+		$this->assertSame( 'invalid_interval', $null_event->get_error_code() );
+
+		$null_rescheduled_event = wp_reschedule_event( time(), 'non_numeric_interval_null', 'hook', array(), true );
+		$this->assertWPError( $null_rescheduled_event );
+		$this->assertSame( 'invalid_interval', $null_rescheduled_event->get_error_code() );
+	}
+
 	/**
 	 * @ticket 49961
 	 *