diff --git a/index.ts b/index.ts
index 9d1c319..f27dd81 100644
--- a/index.ts
+++ b/index.ts
@@ -105,7 +105,7 @@ export default (options?: PluginOptions): Plugin => {
 
 function generateIdentity(source: Buffer, alg: string) {
   const hash = createHash(alg).update(source).digest().toString('base64')
-  return `${alg}-${hash}`
+  return `${alg.toLowerCase()}-${hash}`
 }
 
 function isHtmlAsset(obj: OutputAsset | OutputChunk): obj is OutputAsset {