Problem getting Interface Descriptor from USB to serial converter

Hi all.
Last month, i programmed PIC24FJ256DA206 microcontroller to gather information from USB keyboard and i completed the task successfully.
Now, i would like to get and send information From and To USB to serial converter which is connected with same PIC 24Fj256DA206 microcontroller.
I have successfully gathered Device Descriptor which is of 18 bytes and Configuration Descriptor of 9 bytes.
While closely looking through the Configuration Descriptor, i came to know that my USB to serial converter is vendor specific.
I, then, tried to get Interface Descriptor of USB to serial converter where i get continuous STALL handshake from device. I have referred CDC specification from USB.ORG, in which they mentioned, Interface Descriptor of CDC is same as that of the standard devices.
This is the command i sent along with setup packet to gather Interface Descriptor. Note: i have set device address and i get positive ack from usb device if i send the setup packet. But, i get STALL while i try to read the Interface descriptor of device.
volatile unsigned char GET_INTERFACE_DESCRIPTOR = {0x80, 0x06, 0x00, 0x04, 0x00, 0x00, 0x09, 0x00}
I dont know what had went wrong. Please help me to overcome this issue.

Since when a keyboard is using communication device class? I would rather expect HID. But anyway, a convenient way to retrieve all information and learn about the expected device handling is to run an USB monitor software on your PC and record the keyboard communication.