diff --git a/library/unwind/src/libunwind.rs b/library/unwind/src/libunwind.rs
index a218910f0ec..0fa11f3761a 100644
--- a/library/unwind/src/libunwind.rs
+++ b/library/unwind/src/libunwind.rs
@@ -27,7 +27,10 @@ pub enum _Unwind_Reason_Code {
 #[cfg(target_arch = "x86")]
 pub const unwinder_private_data_size: usize = 5;
 
-#[cfg(target_arch = "x86_64")]
+#[cfg(all(target_arch = "x86_64", not(target_os = "windows")))]
+pub const unwinder_private_data_size: usize = 2;
+
+#[cfg(all(target_arch = "x86_64", target_os = "windows"))]
 pub const unwinder_private_data_size: usize = 6;
 
 #[cfg(all(target_arch = "arm", not(any(target_os = "ios", target_os = "watchos"))))]