diff --git a/src/wp-includes/functions.php b/src/wp-includes/functions.php
index 0fb2117835..9f229291a7 100644
--- a/src/wp-includes/functions.php
+++ b/src/wp-includes/functions.php
@@ -2662,7 +2662,9 @@ function wp_json_encode( $data, $options = 0, $depth = 512 ) {
 	$json = call_user_func_array( 'json_encode', $args );
 
 	// If json_encode() was successful, no need to do more sanity checking.
-	if ( false !== $json ) {
+	// ... unless we're in an old version of PHP, and json_encode() returned
+	// a string containing 'null'. Then we need to do more sanity checking.
+	if ( false !== $json && ( version_compare( PHP_VERSION, '5.5', '>=' ) || false === strpos( $json, 'null' ) ) )  {
 		return $json;
 	}
 
diff --git a/tests/phpunit/tests/functions.php b/tests/phpunit/tests/functions.php
index 7b2870c6e5..1edd143fbc 100644
--- a/tests/phpunit/tests/functions.php
+++ b/tests/phpunit/tests/functions.php
@@ -554,15 +554,7 @@ class Tests_Functions extends WP_UnitTestCase {
 
 		$this->assertEquals( 'aあb', $utf8 );
 
-		// json_encode() returns different things in different PHP versions.
-		// See: https://core.trac.wordpress.org/ticket/30471
-		if ( version_compare( PHP_VERSION, '5.5', '>=' ) ) {
-			$expected = '"a\u3042b"';
-		} else {
-			$expected = 'null';
-		}
-
-		$this->assertEquals( $expected, wp_json_encode( $eucjp ) );
+		$this->assertEquals( '"a\u3042b"', wp_json_encode( $eucjp ) );
 
 		mb_detect_order( $old_charsets );
 	}
@@ -582,15 +574,7 @@ class Tests_Functions extends WP_UnitTestCase {
 
 		$this->assertEquals( 'aあb', $utf8 );
 
-		// json_encode() returns different things in different PHP versions.
-		// See: https://core.trac.wordpress.org/ticket/30471
-		if ( version_compare( PHP_VERSION, '5.5', '>=' ) ) {
-			$expected = '["c","a\u3042b"]';
-		} else {
-			$expected = '["c",null]';
-		}
-
-		$this->assertEquals( $expected, wp_json_encode( array( 'c', $eucjp ) ) );
+		$this->assertEquals( '["c","a\u3042b"]', wp_json_encode( array( 'c', $eucjp ) ) );
 
 		mb_detect_order( $old_charsets );
 	}