diff --git a/releasenotes.md b/releasenotes.md
index f8b2cc771..d3724f558 100644
--- a/releasenotes.md
+++ b/releasenotes.md
@@ -7,6 +7,7 @@
 
 ### Fixes
 - List.remove_at would incorrectly trigger ASAN.
+- With avx512, passing a 512 bit vector in a union would be lowered incorrectly, causing an assert. #2362
 
 ### Stdlib changes
 
diff --git a/src/compiler/abi/c_abi_x64.c b/src/compiler/abi/c_abi_x64.c
index 744c3d799..719aebcf2 100644
--- a/src/compiler/abi/c_abi_x64.c
+++ b/src/compiler/abi/c_abi_x64.c
@@ -207,17 +207,9 @@ void x64_classify_post_merge(ByteSize size, X64Class *lo_class, X64Class *hi_cla
 	// If X87UP is not before X87 => mem
 	// If size > 16 && first isn't SSE or any other is not SSEUP => mem
 	// If SSEUP is not preceded by SSE/SSEUP => convert to SSE.
-	if (*hi_class == CLASS_MEMORY) goto DEFAULT_TO_MEMORY;
-	if (size > 16 && (*lo_class != CLASS_SSE || *hi_class != CLASS_SSEUP)) goto DEFAULT_TO_MEMORY;
-	if (*hi_class == CLASS_SSEUP && *lo_class != CLASS_SSE && *lo_class != CLASS_SSEUP)
-	{
-		// This can happen for unions for example
-		*hi_class = CLASS_SSE;
-	}
-	return;
-
-	DEFAULT_TO_MEMORY:
-	*lo_class = CLASS_MEMORY;
+	if (*hi_class == CLASS_MEMORY) *lo_class = CLASS_MEMORY;
+	if (size > 16 && (*lo_class != CLASS_SSE || *hi_class != CLASS_SSEUP)) *lo_class = CLASS_MEMORY;
+	if (*hi_class == CLASS_SSEUP && *lo_class != CLASS_SSE) *hi_class = CLASS_SSE;
 }
 
 void x64_classify_struct_union(Type *type, ByteSize offset_base, X64Class *current, X64Class *lo_class, X64Class *hi_class, NamedArgument named_arg)
diff --git a/test/test_suite/abi/merge_union_bool_avx512.c3t b/test/test_suite/abi/merge_union_bool_avx512.c3t
new file mode 100644
index 000000000..2534ba66e
--- /dev/null
+++ b/test/test_suite/abi/merge_union_bool_avx512.c3t
@@ -0,0 +1,41 @@
+// #target: macos-x64
+// #opt: --x86cpu=avx512
+module abi;
+
+union Mask64
+{
+    bool[<64>] m;
+    ulong ul;
+}
+
+fn Mask64 make_mask(ulong n)
+{
+    Mask64 mask;
+    mask.ul = n;
+    return mask;
+}
+
+fn int main()
+{
+    make_mask(20);
+    return 0;
+}
+
+/* #expect: abi.ll
+
+define void @abi.make_mask(ptr noalias sret(%Mask64) align 64 %0, i64 %1) #0 {
+entry:
+  %mask = alloca %Mask64, align 64
+  call void @llvm.memset.p0.i64(ptr align 64 %mask, i8 0, i64 64, i1 false)
+  store i64 %1, ptr %mask, align 64
+  call void @llvm.memcpy.p0.p0.i32(ptr align 64 %0, ptr align 64 %mask, i32 64, i1 false)
+  ret void
+}
+
+; Function Attrs: nounwind uwtable
+define i32 @main() #0 {
+entry:
+  %sretparam = alloca %Mask64, align 64
+  call void @abi.make_mask(ptr sret(%Mask64) align 64 %sretparam, i64 20)
+  ret i32 0
+}