diff --git a/src/util.c b/src/util.c
index 86b19831..e18a91b6 100644
--- a/src/util.c
+++ b/src/util.c
@@ -47,7 +47,7 @@ void *alloca (size_t);
 
 #ifdef WIN32
 FILE *fopen(const char *fname, const char *mode) {
-  size_t sz = MultiByteToWideChar(CP_UTF8, 0, fname, -1, NULL, 0);
+  size_t sz = sizeof(wchar_t) * MultiByteToWideChar(CP_UTF8, 0, fname, -1, NULL, 0);
   wchar_t *wfname = alloca(sz);
   MultiByteToWideChar(CP_UTF8, 0, fname, -1, wfname, sz);