diff --git a/lib/plugins/esbuild/index.js b/lib/plugins/esbuild/index.js
index 9cde54556..9706287e5 100644
--- a/lib/plugins/esbuild/index.js
+++ b/lib/plugins/esbuild/index.js
@@ -142,7 +142,7 @@ class Esbuild {
         return true
       }
 
-      const functionName = path.extname(functionHandler).slice(1);
+      const functionName = path.extname(functionObject[handlerPropertyName]).slice(1);
       const handlerPath = functionHandler.replace(`.${functionName}`, '');
       let parsedExtension = undefined;
       for (const extension of ['.js', '.ts', '.cjs', '.mjs', '.cts', '.mts', '.jsx', '.tsx']) {