diff --git a/build/lib/compilation.js b/build/lib/compilation.js
index 170e086b35b7cc173e3f806265280e4a3a623878..59bf1a250f61f5f4a00173981c68e5d4ca0c7eb3 100644
--- a/build/lib/compilation.js
+++ b/build/lib/compilation.js
@@ -44,7 +44,7 @@ function createCompile(src, build, emitError) {
         const input = es.through();
         const output = input
             .pipe(utf8Filter)
-            .pipe(bom())
+            .pipe(bom()) // this is required to preserve BOM in test files that loose it otherwise
             .pipe(utf8Filter.restore)
             .pipe(tsFilter)
             .pipe(util.loadSourcemaps())