Use of constants in formulae supplied as arguments to mxConstraint

It seems that constants cannot be used in the arguments to mxConstraint function calls. Good news, only the first and third arguments need be fixed! Here is an example snippet:

> onelocus<-mxModel("onelocus", mxAlgebraObjective("NegativeLogLikelihood"),

mxMatrix("Full", nrow=1, ncol=1, free=TRUE, values=c(.3333), name="P"), # P, freq of allele 1
mxMatrix("Full", nrow=1, ncol=1, free=TRUE, values=c(.3333), name="Q"), # Q, freq of allele 2
mxMatrix("Full", nrow=1, ncol=1, free=TRUE, values=c(.3333), name="R"), # R, freq of allele 3
mxMatrix("Unit", nrow=1, ncol=1, name="I"), # 1.0 constant for equality constraint
mxMatrix("Full", nrow=4, ncol=1, values=c(234,261,182,94),name="ObservedFreqs"), # Data

mxAlgebra(rbind(P*(P+2*R), Q*(Q+2*R), 2*P*Q, R*R), name="ExpectedFreqs"), # Predicted proportions
mxAlgebra(P+Q+R, name="SumAlleleFreqs"), # P+Q+R has to equal 1.0
mxAlgebra(-(sum( log(ExpectedFreqs) * ObservedFreqs )), name = "NegativeLogLikelihood"), # here is -log Likelihood

mxConstraint("I", "=", "SumAlleleFreqs") # This is the equality constraint
)

#run the model
run<-mxRun(onelocus)
run@matrices
run@algebras

Running onelocus
Error: Unknown reference '1.0' detected in the entity 'untitled4' in model 'onelocus'