diff --git a/lib/index.js b/lib/index.js
index 26cab79..fafe024 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -8,7 +8,7 @@ const options = {
   moduleResolution: ts.ModuleResolutionKind.NodeJs
 };
 
-const prettifyDiagnostic = diagnostic => {
+function prettifyDiagnostic(diagnostic) {
   const { line, character } = diagnostic.file.getLineAndCharacterOfPosition(diagnostic.start);
   const messageText = ts.flattenDiagnosticMessageText(diagnostic.messageText, '\n');