diff --git a/command.php b/command.php
index f4be8b6..db59337 100644
--- a/command.php
+++ b/command.php
@@ -185,7 +185,7 @@ private function themecheck( $theme, $path, $theme_object ) {
 					}
 
 					$error = $this->format_themecheck_result( $error, $log_level );
-					if ( ! in_array( $error, $stack_errors[ $log_level ] ) ) {
+					if ( isset( $stack_errors[ $log_level ] ) && ! in_array( $error, $stack_errors[ $log_level ], true ) ) {
 						$stack_errors[ $log_level ][] = $error;
 					}
 				}