DER format in TLS certificates

Hi, Can I use Der format for certificates? When I try to use DER certificates I get this error on freeradius: (8) eap_tls: ERROR: (TLS) Alert write:fatal:decode error (8) eap_tls: ERROR: (TLS) Server : Error in error (8) eap_tls: ERROR: (TLS) Failed reading from OpenSSL: error:1417C087:SSL routines:tls_process_client_certificate:cert length mismatch (8) eap_tls: ERROR: (TLS) System call (I/O) error (-1) (8) eap_tls: ERROR: (TLS) EAP Receive handshake failed during operation (8) eap_tls: ERROR: [eaptls process] = fail (8) eap: ERROR: Failed continuing EAP TLS (13) session. EAP sub-module failed Thanks in advance! Iñigo