diff --git a/p11-kit/rpc-client.c b/p11-kit/rpc-client.c
index 93e8c7ad..fb39103e 100644
--- a/p11-kit/rpc-client.c
+++ b/p11-kit/rpc-client.c
@@ -430,9 +430,13 @@ 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 mechanism is used for C_*Init () functions to
+	 * cancel operation.  We use a special value 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-message.c b/p11-kit/rpc-message.c
index cf6095ca..09d7f33d 100644
--- a/p11-kit/rpc-message.c
+++ b/p11-kit/rpc-message.c
@@ -2114,8 +2114,14 @@ p11_rpc_buffer_get_mechanism (p11_buffer *buffer,
 
 	mech->mechanism = mechanism;
 
-	/* special NULL case */
-	if (mechanism == 0) {
+	/*
+	 * The NULL mechanism is used for C_*Init () functions to
+	 * cancel operation.  We use a special value 0xffffffff as a
+	 * marker to indicate that.
+	 */
+	if (mechanism == 0xffffffff) {
+		mech->ulParameterLen = 0;
+		mech->pParameter = NULL;
 		return true;
 	}
 
diff --git a/p11-kit/rpc-server.c b/p11-kit/rpc-server.c
index 87bd5744..b4cbcd09 100644
--- a/p11-kit/rpc-server.c
+++ b/p11-kit/rpc-server.c
@@ -480,8 +480,14 @@ proto_read_mechanism (p11_rpc_message *msg,
 		return PARSE_ERROR;
 	}
 
-	if (temp.mechanism == 0) {
+	/*
+	 * The NULL mechanism is used for C_*Init () functions to
+	 * cancel operation.  We use a special value 0xffffffff as a
+	 * marker to indicate that.
+	 */
+	if (temp.mechanism == 0xffffffff) {
 		*mech = NULL;
+		msg->parsed = offset;
 		return CKR_OK;
 	}
 
diff --git a/p11-kit/test-rpc.c b/p11-kit/test-rpc.c
index 130d50ee..f2145092 100644
--- a/p11-kit/test-rpc.c
+++ b/p11-kit/test-rpc.c
@@ -675,6 +675,31 @@ test_simultaneous_functions (void *module)
 	p11_mutex_uninit (&delay_mutex);
 }
 
+static void
+test_mechanism_unsupported (void *module)
+{
+	CK_FUNCTION_LIST_PTR rpc_module;
+	CK_SESSION_HANDLE session;
+	CK_MECHANISM mech;
+	CK_RV rv;
+
+	rpc_module = setup_test_rpc_module (&test_normal_vtable,
+	                                    module, &session);
+
+	memset (&mech, 0, sizeof(mech));
+
+	/*
+	 * This mechanism is not supported by the remote mock module,
+	 * but it should be able to return an error through RPC.
+	 */
+	mech.mechanism = CKM_RSA_PKCS_KEY_PAIR_GEN;
+
+	rv = (rpc_module->C_DigestInit) (session, &mech);
+	assert_num_eq (rv, CKR_MECHANISM_INVALID);
+
+	teardown_mock_module (rpc_module);
+}
+
 #ifdef OS_UNIX
 
 static void
@@ -759,6 +784,7 @@ main (int argc,
 	p11_testx (test_get_info_stand_in, &mock_module_no_slots, "/rpc/get-info-stand-in");
 	p11_testx (test_get_slot_list_no_device, &mock_module_no_slots, "/rpc/get-slot-list-no-device");
 	p11_testx (test_simultaneous_functions, &mock_module_no_slots, "/rpc/simultaneous-functions");
+	p11_testx (test_mechanism_unsupported, &mock_module, "/rpc/mechanism-unsupported");
 
 #ifdef OS_UNIX
 	p11_testx (test_fork_and_reinitialize, &mock_module_no_slots, "/rpc/fork-and-reinitialize");
@@ -778,6 +804,7 @@ main (int argc,
 	p11_testx (test_get_info_stand_in, &mock_module_v3_no_slots, "/rpc3/get-info-stand-in");
 	p11_testx (test_get_slot_list_no_device, &mock_module_v3_no_slots, "/rpc3/get-slot-list-no-device");
 	p11_testx (test_simultaneous_functions, &mock_module_v3_no_slots, "/rpc3/simultaneous-functions");
+	p11_testx (test_mechanism_unsupported, &mock_module_v3, "/rpc3/mechanism-unsupported");
 
 #ifdef OS_UNIX
 	p11_testx (test_fork_and_reinitialize, &mock_module_v3_no_slots, "/rpc3/fork-and-reinitialize");