diff --git a/lib/BannerPlugin.js b/lib/BannerPlugin.js
index f0e6cc354..f4d364e2c 100644
--- a/lib/BannerPlugin.js
+++ b/lib/BannerPlugin.js
@@ -93,9 +93,10 @@ class BannerPlugin {
 
 						const lastSlashIndex = filename.lastIndexOf("/");
 
-						const basename = lastSlashIndex
-							? filename
-							: filename.substr(lastSlashIndex + 1);
+						const basename =
+							lastSlashIndex === -1
+								? filename
+								: filename.substr(lastSlashIndex + 1);
 
 						const data = {
 							hash,