R Error: System is computationally singular

Greetings,

I am trying to fit a zero-inflated Poisson model using zeroinfl()  
from the
pscl library. I have 5 covariates (4 continuous, 1 categorical); the
categorical variable has 7 levels.  I have had success fitting  
models that
contain only the continuous covariates; however, when I add the  
categorical
variable to any of the models (or if I run it by itself) I get the  
following
error:

Error in solve.default(as.matrix(fit$hessian)) :

 system is computationally singular: reciprocal condition number =
3.46934e-20

The code I am using is:

library(pscl)
f1 <- formula(LOCS ~ as.factor(LCOVER) + D_ROADS + D_WATER + D_EDGE +
D_GRASS)
ZIP1 <- zeroinfl(f1, dist="poisson", link = "logit", data = FAWNS)

There is no correlation between my covariates. Also, I tried  
reducing my
categorical covariate to 3 levels and still receive the same error.  
Can
anyone suggest why I may be getting this error when I add the  
categorical
covariate?