diff --git a/src/wp-includes/rest-api/class-wp-rest-server.php b/src/wp-includes/rest-api/class-wp-rest-server.php
index 68be073eef..7f158959e9 100644
--- a/src/wp-includes/rest-api/class-wp-rest-server.php
+++ b/src/wp-includes/rest-api/class-wp-rest-server.php
@@ -405,6 +405,11 @@ class WP_REST_Server {
 			 */
 			$result = apply_filters( 'rest_pre_echo_response', $result, $this, $request );
 
+			// The 204 response shouldn't have a body.
+			if ( 204 === $code || null === $result ) {
+				return null;
+			}
+
 			$result = wp_json_encode( $result );
 
 			$json_error_message = $this->get_json_last_error();
diff --git a/tests/phpunit/tests/rest-api/rest-server.php b/tests/phpunit/tests/rest-api/rest-server.php
index 8c514e9e66..efb656ed4e 100644
--- a/tests/phpunit/tests/rest-api/rest-server.php
+++ b/tests/phpunit/tests/rest-api/rest-server.php
@@ -1144,6 +1144,48 @@ class Tests_REST_Server extends WP_Test_REST_TestCase {
 		$this->assertEquals( 200, $response->get_status() );
 	}
 
+	/**
+	 * @ticket 43691
+	 */
+	public function test_does_not_echo_body_for_null_responses() {
+		register_rest_route(
+			'test-ns',
+			'/test',
+			array(
+				'methods'  => array( 'GET' ),
+				'callback' => function () {
+					return new WP_REST_Response();
+				},
+			)
+		);
+
+		$result = rest_get_server()->serve_request( '/test-ns/test' );
+
+		$this->assertNull( $result );
+		$this->assertEquals( '', rest_get_server()->sent_body );
+	}
+
+	/**
+	 * @ticket 43691
+	 */
+	public function test_does_not_echo_body_for_responses_with_204_status() {
+		register_rest_route(
+			'test-ns',
+			'/test',
+			array(
+				'methods'  => array( 'GET' ),
+				'callback' => function () {
+					return new WP_REST_Response( 'data', 204 );
+				},
+			)
+		);
+
+		$result = rest_get_server()->serve_request( '/test-ns/test' );
+
+		$this->assertNull( $result );
+		$this->assertEquals( '', rest_get_server()->sent_body );
+	}
+
 	public function _validate_as_integer_123( $value, $request, $key ) {
 		if ( ! is_int( $value ) ) {
 			return new WP_Error( 'some-error', 'This is not valid!' );