diff --git a/src/util.c b/src/util.c
index 3f7cab8d..7eefc666 100644
--- a/src/util.c
+++ b/src/util.c
@@ -48,11 +48,11 @@ void *alloca (size_t);
 #ifdef WIN32
 FILE *fopen(const char *fname, const char *mode) {
   size_t sz = sizeof(wchar_t) * MultiByteToWideChar(CP_UTF8, 0, fname, -1, NULL, 0);
-  wchar_t *wfname = alloca(sz);
+  wchar_t *wfname = alloca(sz + 2); // +2 is not needed, but just in case
   MultiByteToWideChar(CP_UTF8, 0, fname, -1, wfname, sz);
 
-  sz = MultiByteToWideChar(CP_UTF8, 0, mode, -1, NULL, 0);
-  wchar_t *wmode = alloca(sz);
+  sz = sizeof(wchar_t) * MultiByteToWideChar(CP_UTF8, 0, mode, -1, NULL, 0);
+  wchar_t *wmode = alloca(sz); // +2 is not needed, but just in case
   MultiByteToWideChar(CP_UTF8, 0, mode, -1, wmode, sz);
   return _wfopen(wfname, wmode);
 }