diff --git a/random.c b/random.c
index 4734cf748b..82111ce69e 100644
--- a/random.c
+++ b/random.c
@@ -485,7 +485,7 @@ fill_random_bytes(void *seed, size_t size)
 	if (!CryptAcquireContext(&prov, NULL, NULL, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT)) {
 	    prov = (HCRYPTPROV)INVALID_HANDLE_VALUE;
 	}
-	old_prov = (HCRYPTPROV)ATOMIC_PTR_CAS(perm_prov, 0, prov);
+	old_prov = (HCRYPTPROV)ATOMIC_SIZE_CAS(perm_prov, 0, prov);
 	if (prov == (HCRYPTPROV)INVALID_HANDLE_VALUE) {
 	    if (old_prov) prov = old_prov;
 	}