diff --git a/p11-kit/rpc-client.c b/p11-kit/rpc-client.c
index 93e8c7ad..0a9208ba 100644
--- a/p11-kit/rpc-client.c
+++ b/p11-kit/rpc-client.c
@@ -430,9 +430,12 @@ proto_write_mechanism (p11_rpc_message *msg,
 	/* Make sure this is in the right order */
 	assert (!msg->signature || p11_rpc_message_verify_part (msg, "M"));
 
-	/* This case is valid for C_*Init () functions to cancel operation */
+	/*
+	 * The NULL mech is used for C_*Init () functions to cancel operation.
+	 * We use 0xffffffff as a marker to indicate that.
+	 */
 	if (mech == NULL) {
-		p11_rpc_buffer_add_uint32 (msg->output, 0);
+		p11_rpc_buffer_add_uint32 (msg->output, 0xffffffff);
 		return p11_buffer_failed (msg->output) ? CKR_HOST_MEMORY : CKR_OK;
 	}
 
diff --git a/p11-kit/rpc-server.c b/p11-kit/rpc-server.c
index 87bd5744..bdaded11 100644
--- a/p11-kit/rpc-server.c
+++ b/p11-kit/rpc-server.c
@@ -480,8 +480,13 @@ proto_read_mechanism (p11_rpc_message *msg,
 		return PARSE_ERROR;
 	}
 
-	if (temp.mechanism == 0) {
+	/*
+	 * The NULL mech is used for C_*Init () functions to cancel operation.
+	 * We use 0xffffffff as a marker to indicate that.
+	 */
+	if (temp.mechanism == 0xffffffff) {
 		*mech = NULL;
+		msg->parsed = offset;
 		return CKR_OK;
 	}