Calling "core_user_get_users" function using REST API

I am using Moodle Rest API in java to call core_user_get_users function and it is displaying the following error,
<EXCEPTION class="invalid_parameter_exception">
<ERRORCODE>invalidparameter</ERRORCODE>
<MESSAGE>Invalid parameter value detected</MESSAGE>
<DEBUGINFO>Missing required key in single structure: criteria</DEBUGINFO>
</EXCEPTION>

I tried to give criteria[0]=email, but it gave me below error,

criteria => Invalid parameter value detected: Only arrays accepted. The bad value is: 'email'

It would be highly appreciated if someone could help me in how to pass the criteria parameter ?