diff --git a/src/libsodium/randombytes/sysrandom/randombytes_sysrandom.c b/src/libsodium/randombytes/sysrandom/randombytes_sysrandom.c
index 304f4878..d0e25209 100644
--- a/src/libsodium/randombytes/sysrandom/randombytes_sysrandom.c
+++ b/src/libsodium/randombytes/sysrandom/randombytes_sysrandom.c
@@ -169,7 +169,10 @@ randombytes_sysrandom_buf(void * const buf, const size_t size)
         abort();
     }
 #else
-    if (! CryptGenRandom(stream.hcrypt_prov, size, (BYTE *) buf)) {
+    if (size > 0xffffffff) {
+        abort();
+    }
+    if (! CryptGenRandom(stream.hcrypt_prov, (DWORD) size, (BYTE *) buf)) {
         abort();
     }
 #endif