diff --git a/testing/fx_string_testhelpers.cpp b/testing/fx_string_testhelpers.cpp
index 17d90cc..4a7bda7 100644
--- a/testing/fx_string_testhelpers.cpp
+++ b/testing/fx_string_testhelpers.cpp
@@ -42,7 +42,7 @@
 
 std::wstring GetPlatformWString(FPDF_WIDESTRING wstr) {
   if (!wstr)
-    return nullptr;
+    return std::wstring();
 
   size_t characters = 0;
   while (wstr[characters])