diff --git a/pkgs/by-name/fo/folly/memset-benchmark-darwin.patch b/pkgs/by-name/fo/folly/memset-benchmark-darwin.patch
new file mode 100644
index 000000000000..218789a22acb
--- /dev/null
+++ b/pkgs/by-name/fo/folly/memset-benchmark-darwin.patch
@@ -0,0 +1,11 @@
+--- a/folly/test/MemsetBenchmark.cpp
++++ b/folly/test/MemsetBenchmark.cpp
+@@ -40,7 +40,7 @@
+ 
+ template <void* memset_impl(void*, int, size_t)>
+ void bmMemset(void* buf, size_t length, size_t iters) {
+-#if !defined(__aarch64__)
++#if !defined(__aarch64__) && !defined(__APPLE__)
+   __asm__ volatile(".align 64\n");
+ #endif
+ #pragma unroll(1)
diff --git a/pkgs/by-name/fo/folly/package.nix b/pkgs/by-name/fo/folly/package.nix
index 196179c9e423..379312d79ad4 100644
--- a/pkgs/by-name/fo/folly/package.nix
+++ b/pkgs/by-name/fo/folly/package.nix
@@ -144,6 +144,11 @@ stdenv.mkDerivation (finalAttrs: {
     # https://github.com/facebook/folly/pull/2561
     ./memset-memcpy-aarch64.patch
 
+    # `.align 64` is invalid on x86_64 Mach-O, where `.align` takes a
+    # power-of-two exponent (64 means 2^64). The guard only excluded
+    # aarch64, so add !__APPLE__ to also skip x86_64-darwin.
+    ./memset-benchmark-darwin.patch
+
     # Use feature detection directly instead of private standard library
     # macros to detect the presence of ASAN and otherwise fallback to
     # _not_ having ASAN.