diff --git a/src/wp-admin/js/customize-controls.js b/src/wp-admin/js/customize-controls.js
index 2d13952a42..5ff47c4998 100644
--- a/src/wp-admin/js/customize-controls.js
+++ b/src/wp-admin/js/customize-controls.js
@@ -3202,7 +3202,7 @@
 			}
 
 			// Prevent loading a non-active theme preview when there is a drafted/scheduled changeset.
-			if ( panel.canSwitchTheme( slug ) ) {
+			if ( ! panel.canSwitchTheme( slug ) ) {
 				deferred.reject({
 					errorCode: 'theme_switch_unavailable'
 				});
@@ -3299,7 +3299,10 @@
 
 			// Prevent loading a non-active theme preview when there is a drafted/scheduled changeset.
 			if ( ! panel.canSwitchTheme( themeId ) ) {
-				return deferred.reject().promise();
+				deferred.reject({
+					errorCode: 'theme_switch_unavailable'
+				});
+				return deferred.promise();
 			}
 
 			urlParser = document.createElement( 'a' );
diff --git a/src/wp-includes/class-wp-customize-manager.php b/src/wp-includes/class-wp-customize-manager.php
index 607349b04a..bb4aafb0b7 100644
--- a/src/wp-includes/class-wp-customize-manager.php
+++ b/src/wp-includes/class-wp-customize-manager.php
@@ -5446,7 +5446,12 @@ final class WP_Customize_Manager {
 				$theme->id           = $theme->slug;
 				$theme->screenshot   = array( $theme->screenshot_url );
 				$theme->authorAndUri = $theme->author;
-				$theme->parent       = ( $theme->slug === $theme->template ) ? false : $theme->template; // The .org API does not seem to return the parent in a documented way; however, this check should yield a similar result in most cases.
+				// The .org API can return the full parent theme details if passed the 'parent' arg, or if passed the 'template' option it'll return that in the event it's a child theme.
+				if ( isset( $theme->parent ) ) {
+					$theme->parent = $theme->parent['slug'];
+				} else {
+					$theme->parent = false;
+				}
 				unset( $theme->slug );
 				unset( $theme->screenshot_url );
 				unset( $theme->author );