Problems with vegdist in Vegan

Dear list members,

I am runnig a NMDS in Vegan (version 1.13.2, Windows) and I get the 
following message with the function vegdist:

 > aves.dis <- vegdist(avesp.esp, dist="jaccard")
Warning messages:
1: In vegdist(avesp.esp, dist = "jaccard") :
  you have empty rows: their dissimilarities may be meaningless in 
method bray

2: In vegdist(avesp.esp, dist = "jaccard") : missing values in results

I have sites (point counts) without species, is it a problem?
Thank you very much in advance.
Best,

Manuel Sp?nola

Manuel Sp?nola, Ph.D.
Instituto Internacional en Conservaci?n y Manejo de Vida Silvestre
Universidad Nacional
Apartado 1350-3000
Heredia
COSTA RICA
mspinola at una.ac.cr
mspinola10 at gmail.com
Tel?fono: 277-3598
Fax: 237-7036