diff --git a/c/blake3_dispatch.c b/c/blake3_dispatch.c
index 5c76b14b..f04f5a08 100644
--- a/c/blake3_dispatch.c
+++ b/c/blake3_dispatch.c
@@ -234,7 +234,7 @@ void blake3_xof_many(const uint32_t cv[8],
   }
 #if defined(IS_X86)
   const enum cpu_feature features = get_cpu_features();
-#if defined(__unix__) && !defined(BLAKE3_NO_AVX512)
+#if !defined(_WIN32) && !defined(BLAKE3_NO_AVX512)
   if (features & AVX512VL) {
     blake3_xof_many_avx512(cv, block, block_len, counter, flags, out, outblocks);
     return;
diff --git a/c/blake3_impl.h b/c/blake3_impl.h
index 3bb91691..51d792a8 100644
--- a/c/blake3_impl.h
+++ b/c/blake3_impl.h
@@ -283,7 +283,7 @@ void blake3_hash_many_avx512(const uint8_t *const *inputs, size_t num_inputs,
                              uint8_t flags, uint8_t flags_start,
                              uint8_t flags_end, uint8_t *out);
 
-#if defined(__unix__)
+#if !defined(_WIN32)
 void blake3_xof_many_avx512(const uint32_t cv[8],
                             const uint8_t block[BLAKE3_BLOCK_LEN],
                             uint8_t block_len, uint64_t counter, uint8_t flags,