diff --git a/src/string.rs b/src/string.rs
index 907032d..8635d10 100644
--- a/src/string.rs
+++ b/src/string.rs
@@ -501,6 +501,7 @@ mod inner {
 mod inner {
     use super::*;
 
+    #[repr(C)]
     pub(super) union KStringInner<B> {
         tag: TagVariant,
         singleton: SingletonVariant,