diff --git a/src/wp-admin/includes/class-theme-upgrader.php b/src/wp-admin/includes/class-theme-upgrader.php
index 466bb29796..4ff5109e63 100644
--- a/src/wp-admin/includes/class-theme-upgrader.php
+++ b/src/wp-admin/includes/class-theme-upgrader.php
@@ -94,16 +94,18 @@ class Theme_Upgrader extends WP_Upgrader {
 		/* translators: %s: Theme error. */
 		$this->strings['current_theme_has_errors'] = __( 'The current theme has the following error: "%s".' );
 
-		if ( 'update-theme' === $this->skin->overwrite ) {
-			$this->strings['installing_package'] = __( 'Updating the theme…' );
-			$this->strings['process_failed']     = __( 'Theme update failed.' );
-			$this->strings['process_success']    = __( 'Theme updated successfully.' );
-		}
+		if ( ! empty( $this->skin->overwrite ) ) {
+			if ( 'update-theme' === $this->skin->overwrite ) {
+				$this->strings['installing_package'] = __( 'Updating the theme…' );
+				$this->strings['process_failed']     = __( 'Theme update failed.' );
+				$this->strings['process_success']    = __( 'Theme updated successfully.' );
+			}
 
-		if ( 'downgrade-theme' === $this->skin->overwrite ) {
-			$this->strings['installing_package'] = __( 'Downgrading the theme…' );
-			$this->strings['process_failed']     = __( 'Theme downgrade failed.' );
-			$this->strings['process_success']    = __( 'Theme downgraded successfully.' );
+			if ( 'downgrade-theme' === $this->skin->overwrite ) {
+				$this->strings['installing_package'] = __( 'Downgrading the theme…' );
+				$this->strings['process_failed']     = __( 'Theme downgrade failed.' );
+				$this->strings['process_success']    = __( 'Theme downgraded successfully.' );
+			}
 		}
 	}