OpenCart api login return empty string instead of success or fail message
<p>I had created a test store in OpenCart. I try to login the api by <a href="http://opencart.frontierforce.com/index.php?route=api/login" rel="nofollow noreferrer">http://opencart.frontierforce.com/index.php?route=api/login</a>, but i got the result like this:</p>

<p><a href="https://i.stack.imgur.com/aBuh5.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/aBuh5.png" alt="enter image description here"></a>
I tried the same for their demo api and it is returning the correct message. </p>

<p>I have checked on the setting on API, user, user permission and allow my IP to access on the API, but I still cannot get the correct message. Can anyone help me on this? Many thanks in advanced!!!</p>