diff --git a/unittests/Tooling/RewriterTestContext.h b/unittests/Tooling/RewriterTestContext.h
index edcf77c..73f9aa6 100644
--- a/unittests/Tooling/RewriterTestContext.h
+++ b/unittests/Tooling/RewriterTestContext.h
@@ -85,6 +85,7 @@
     std::string Result;
     llvm::raw_string_ostream OS(Result);
     Rewrite.getEditBuffer(ID).write(OS);
+    OS.flush();
     return Result;
   }