JQuery ajaxHandleResponse() throwing undefined is not a function

after sending an AJAX request using JQuery I get the error : undefined is not a function although the request was successful and I get the response from server at browser developer tools.

after trying some debugging I found the problem which lays in this code:

function ajaxHandleResponses( s, jqXHR, responses ) {

var ct, type, finalDataType, firstDataType,
    contents = s.contents,
    dataTypes = s.dataTypes;

// Remove auto dataType and get content-type in the process
while ( dataTypes[ 0 ] === "*" ) {
    dataTypes.shift();
    if ( ct === undefined ) {
        ct = s.mimeType || jqXHR.getResponseHeader("Content-Type");
    }
}

// Check if we're dealing with a known content-type
if ( ct ) {
    for ( type in contents ) {
        if ( contents[ type ] && contents[ type ].test( ct ) ) { // HERE IS WHERE THE EXCEPTION IS TRIGGERED
            dataTypes.unshift( type );
            break;
        }
    }
}

// Check to see if we have a response for the expected dataType
if ( dataTypes[ 0 ] in responses ) {
    finalDataType = dataTypes[ 0 ];
} else {
    // Try convertible dataTypes
    for ( type in responses ) {
        if ( !dataTypes[ 0 ] || s.converters[ type + " " + dataTypes[0] ] ) {
            finalDataType = type;
            break;
        }
        if ( !firstDataType ) {
            firstDataType = type;
        }
    }
    // Or just use first one
    finalDataType = finalDataType || firstDataType;
}

// If we found a dataType
// We add the dataType to the list if needed
// and return the corresponding response
if ( finalDataType ) {
    if ( finalDataType !== dataTypes[ 0 ] ) {
        dataTypes.unshift( finalDataType );
    }
    return responses[ finalDataType ];
}

}

the undefined function is test which is called at contents[type].test(ct), I believe that contents[type] should be a regex which is not and it's value in my case is "json" , a String.

anyone got any clue about this? :) thx in advance