diff --git a/test/util.h b/test/util.h
index 803cdeea..dee1d415 100644
--- a/test/util.h
+++ b/test/util.h
@@ -9,6 +9,12 @@
 #include <cstdio>
 #include <locale>
 #include <string>
+#include <stdexcept>
+#include <cerrno>
+#include <cstring>
+
+
+
 
 #include "fmt/os.h"
 
@@ -31,14 +37,24 @@ extern const char* const file_content;
 // Opens a buffered file for reading.
 auto open_buffered_file(FILE** fp = nullptr) -> fmt::buffered_file;
 
-inline auto safe_fopen(const char* filename, const char* mode) -> FILE* {
+inline FILE* safe_fopen(const char* filename, const char* mode) {
 #if defined(_WIN32) && !defined(__MINGW32__)
-  // Fix MSVC warning about "unsafe" fopen.
   FILE* f = nullptr;
-  errno = fopen_s(&f, filename, mode);
+  int err = fopen_s(&f, filename, mode);
+  if (err != 0 || f == nullptr) {
+    throw std::runtime_error(
+      std::string("cannot open file ") + filename + ": " + std::strerror(err)
+    );
+  }
   return f;
 #else
-  return std::fopen(filename, mode);
+  FILE* f = std::fopen(filename, mode);
+  if (f == nullptr) {
+    throw std::runtime_error(
+      std::string("cannot open file ") + filename + ": " + std::strerror(errno)
+    );
+  }
+  return f;
 #endif
 }