diff --git a/lib/Server.js b/lib/Server.js
index 9cffb872e2..2424af7ec9 100644
--- a/lib/Server.js
+++ b/lib/Server.js
@@ -1972,7 +1972,7 @@ class Server {
           // If key = main.other.mjs && filename = main.other => true
           if (
             splitKey.length - 1 === splitOutputFilename.length &&
-            key.split(".")[0] === outputFilename.split(".")[0]
+            splitKey[0] === outputFilename.split(".")[0]
           ) {
             extension = splitKey[splitKey.length - 1];
             isOutputModule = value.javascriptModule;