diff --git a/rpc/src/v1/types/trace.rs b/rpc/src/v1/types/trace.rs
index abde04b96..f4380b0a3 100644
--- a/rpc/src/v1/types/trace.rs
+++ b/rpc/src/v1/types/trace.rs
@@ -99,6 +99,7 @@ pub struct VMOperation {
 	/// Information concerning the execution of the operation.
 	pub ex: Option<VMExecutedOperation>,
 	/// Subordinate trace of the CALL/CREATE if applicable.
+	#[serde(bound="VMTrace: Serialize")]
 	pub sub: Option<VMTrace>,
 }