diff --git a/src/doc/guide-ffi.md b/src/doc/guide-ffi.md
index 256bcb2949e..0ded9db3ad0 100644
--- a/src/doc/guide-ffi.md
+++ b/src/doc/guide-ffi.md
@@ -263,6 +263,7 @@ Rust code:
 
 ~~~~no_run
 
+#[repr(C)]
 struct RustObject {
     a: i32,
     // other members