Test connection TO UDP PORT 1813

Hello, iam using nc tool to test connection to port 1813 of my freeradius server and iam getting this error on freeradius debug mode: /usr/bin/nc -vnzu freeradiusIP 1813 Receive - Invalid data from NASIP: Expected at least 4 bytes of header data, got 1 bytes any one have an idea how to send 4 bytes of header data with nc tool? Thanks Bassem