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D VelD Vel 

Error on the Apex Callout to the Rest API

I trying to test the Apex Call outs which uses the OAuth Authentication. I am not sure what I am missing here it throws error on the line HttpResponse res1 = http1.send(req1); like 

Line: 22, Column: 1
System.UnexpectedException: java.lang.IllegalArgumentException: invalid start or end
public class TestD365API {
    public final String clientId = 'xxxxxxxxxxxxx';
    public final String clientSecret = 'xxxxxxxx';
    public String getD365Customer(){             
        String reqbody = 'grant_type=client_credentials&client_id='+clientId+'&client_secret='+clientSecret;
        Http h = new Http();
        HttpRequest req = new HttpRequest();
        HttpResponse res = h.send(req);
        deserializeResponse resp1 = (deserializeResponse)JSON.deserialize(res.getbody(),deserializeResponse.class);
        String atoken = resp1.access_token;
        Http http1 = new Http();
        HttpRequest req1 = new HttpRequest();
        req1.setHeader('Authorization','Bearer '+atoken);
        HttpResponse res1 = http1.send(req1);
        System.debug('Response Body=========' + res1.getBody());
        return res1.getBody();   
    public class deserializeResponse
        public String token_type;
        public String expires_in;
        public String ext_expires_in;
        public String expires_on;
        public String not_before;
        public String resource;
        public String access_token;
Please help me fixing this issue.
Suraj MakandarSuraj Makandar
Hi D Vel,

IllegalArgumentException is thrown when an illegal argument was provided to a method call. For example, a method that requires a non-null argument throws this exception if a null value is passed into the method.

Please check all the headers/parameters for you API calls and their values being passed.