could not update an item in the inventory

could not update an item in the inventory

Hi,

I am a developer from Germany. I 'm new to zoho. 
I am able to add an item to the inventory. However, I am mot able to update an item. 

This is how I used to add an item to the inventory. And its successful.

curl https://inventory.zoho.eu/api/v1/items?organization_id=20075964771
-X POST
-H "Authorization: Zoho-oauthtoken 1000.xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
-H "Content-Type: application/json"
-d '{
    "name": "Toothpaste",
}'

I have tried like below to update an item. 

curl https://inventory.zoho.eu/api/v1/items?organization_id=20075964771
-X PUT
-H "Authorization: Zoho-oauthtoken 1000.xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
-H "Content-Type: application/json"
-d '{
    "name": "Toothpaste",
    "initial_stock": 50,
}'

I'm getting an error like below.

{
    "code": 1000,
    "message": "Interner Fehler"
}

means an internal error.  Also I see errors when I run the example from the zoho inventory api. 
Thank you.