diff --git a/programs/mango-v4/src/instructions/serum3_cancel_order.rs b/programs/mango-v4/src/instructions/serum3_cancel_order.rs
index 98fc9c8bd..a08554188 100644
--- a/programs/mango-v4/src/instructions/serum3_cancel_order.rs
+++ b/programs/mango-v4/src/instructions/serum3_cancel_order.rs
@@ -40,6 +40,7 @@ pub struct Serum3CancelOrder<'info> {
 
     // These accounts are forwarded directly to the serum cpi call
     // and are validated there.
+    #[account(mut)]
     /// CHECK: Validated by the serum cpi call
     pub market_bids: UncheckedAccount<'info>,
     #[account(mut)]