[R] Error "system is computationally singular" by using function dmvnorm

[Alla Bulashevska]

Dear R users,
i try to use function dmvnorm(x, mean, sigma, log=FALSE)
from R package mvtnorm to calculate the probability of x
under the multivariate normal distribution with mean equal
to mean and covariance matrix sigma.
I become the following 
Error in solve.default(cov, ...) :
  system is computationally singular: reciprocal condition
number = 1.81093e-19

What could be the reason of it?
Thank you in Advance,
Alla.