[R] Error in structural equation model - "The model has negative degrees of freedom"

[Sunil W]

Hi

I am running a structural equation model with R using the sem command; am 
getting the following error:

"Error in sem.default : The model has negative degrees of freedom = -4"


My model is as follows:

s_model = specify.model()
x1->m1, b1,NA
x2->m1, b2,NA
x3->m2, b3,NA
x4->m2, b4,NA
x5->m2, b5,NA
x6->m2, b6,NA
m1->y, a1,NA
m2->y, a2,NA
m1<->m1, v1,NA
m2<->m2, v2,NA
y<->y, v3,NA


x1-x6 are observed independent variables, m1 and m2 are the latent variables 
and y is the observed dependent variable. I use the raw.moments command for 
calculating the covariance matrix, based on a data with 147 observations.

The command that I use is as follows:

s = sem(s_model,S=R,obs.variables=colnames(R), 
fixed.x=c('x1','x2','x3','x4','x5','x6'), raw=TRUE)


I would appreciate any help on this; I am new to structural equation models 
and realize that I may be making a silly error.

Thanks
Sunil