diff --git a/wp-includes/class-wp-site.php b/wp-includes/class-wp-site.php
index c16f498e90..4bca299ef3 100644
--- a/wp-includes/class-wp-site.php
+++ b/wp-includes/class-wp-site.php
@@ -240,11 +240,15 @@ final class WP_Site {
 			case 'siteurl':
 			case 'post_count':
 			case 'home':
+			default: // Custom properties added by 'site_details' filter.
 				if ( ! did_action( 'ms_loaded' ) ) {
 					return null;
 				}
+
 				$details = $this->get_details();
-				return $details->$key;
+				if ( isset( $details->$key ) ) {
+					return $details->$key;
+				}
 		}
 
 		return null;
@@ -275,6 +279,15 @@ final class WP_Site {
 					return false;
 				}
 				return true;
+			default: // Custom properties added by 'site_details' filter.
+				if ( ! did_action( 'ms_loaded' ) ) {
+					return false;
+				}
+
+				$details = $this->get_details();
+				if ( isset( $details->$key ) ) {
+					return true;
+				}
 		}
 
 		return false;
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 298b94d37e..85933eab86 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.8-alpha-40477';
+$wp_version = '4.8-alpha-40478';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.