diff --git a/app/ibc-hooks/message.go b/app/ibc-hooks/message.go
index 35c738df..b313eefb 100644
--- a/app/ibc-hooks/message.go
+++ b/app/ibc-hooks/message.go
@@ -1,6 +1,8 @@
 package evm_hooks
 
 import (
+	"encoding/json"
+
 	evmtypes "github.com/initia-labs/minievm/x/evm/types"
 )
 
@@ -43,3 +45,41 @@ type HookData struct {
 	// AsyncCallback is a contract address
 	AsyncCallback *AsyncCallback `json:"async_callback,omitempty"`
 }
+
+// asyncCallback is same as AsyncCallback.
+type asyncCallback struct {
+	// callback id should be issued form the executor contract
+	Id              uint64 `json:"id"`
+	ContractAddress string `json:"contract_address"`
+}
+
+// asyncCallbackStringID is same as AsyncCallback but
+// it has Id as string.
+type asyncCallbackStringID struct {
+	// callback id should be issued form the executor contract
+	Id              uint64 `json:"id,string"`
+	ContractAddress string `json:"contract_address"`
+}
+
+// UnmarshalJSON implements the json unmarshaler interface.
+// custom unmarshaler is required because we have to handle
+// id as string and uint64.
+func (a *AsyncCallback) UnmarshalJSON(bz []byte) error {
+	var ac asyncCallback
+	err := json.Unmarshal(bz, &ac)
+	if err != nil {
+		var acStr asyncCallbackStringID
+		err := json.Unmarshal(bz, &acStr)
+		if err != nil {
+			return err
+		}
+
+		a.Id = acStr.Id
+		a.ContractAddress = acStr.ContractAddress
+		return nil
+	}
+
+	a.Id = ac.Id
+	a.ContractAddress = ac.ContractAddress
+	return nil
+}
diff --git a/app/ibc-hooks/message_test.go b/app/ibc-hooks/message_test.go
new file mode 100644
index 00000000..27833179
--- /dev/null
+++ b/app/ibc-hooks/message_test.go
@@ -0,0 +1,30 @@
+package evm_hooks_test
+
+import (
+	"encoding/json"
+	"testing"
+
+	evmhooks "github.com/initia-labs/minievm/app/ibc-hooks"
+	"github.com/stretchr/testify/require"
+)
+
+func Test_Unmarshal_AsyncCallback(t *testing.T) {
+	var callback evmhooks.AsyncCallback
+	err := json.Unmarshal([]byte(`{
+		"id": 99,
+		"contract_address": "0x1"
+	}`), &callback)
+	require.NoError(t, err)
+	require.Equal(t, evmhooks.AsyncCallback{
+		Id:              99,
+		ContractAddress: "0x1",
+	}, callback)
+
+	var callbackStringID evmhooks.AsyncCallback
+	err = json.Unmarshal([]byte(`{
+		"id": "99",
+		"contract_address": "0x1"
+	}`), &callbackStringID)
+	require.NoError(t, err)
+	require.Equal(t, callback, callbackStringID)
+}