Problems creating new_ode model

[lostinpkpd]

Hi guys,

I want to create a one compartment model with first order absorption and elimination (drug administered orally). I want the model to contain bioavailabilty and half life time and to be able to show concentrations and to predict time to reach steady state.

I first tried to model it like this (without halflife time):

par_mod_BA<-list(KA=1, KEL=0.6, F=0.98)
reg_mod_BA<-new_regimen(
amt=400,
interval=24,
n=5
)
mod_BA<-new_ode_model(
code="
dAdt[1]=-KAA[1];
dAdt[2]=KAA[1]-KEL*A[2]",
dose=list(cmt=1, bioav="F"),
obs=list(cmt=2),
parameters=c("F","KEL", "KA")
)

My questions is:
I want to include half life time "HLT", that can be described by the term HLT=ln(2)/KEL, KEL=ln(2)/HLT respectively. I tried it by making HLT a covariate.

cov1<-list("HLT"=new_covariate(value=17))

par_mod_BA<-list(KA=1, KEL=0.6, F=0.98)

reg_mod_BA<-new_regimen(
amt=400,
interval=24,
n=5
)

mod_BA_HLT<-new_ode_model(
code="
dAdt[1]=-KAA[1];
dAdt[2]=KAA[1]-KEL*A[2];
KEL=log(2)/HLT",
declare_variables=c("KEL","KA"),
covariates=cov1
)

data_mod_BA_HLT<-sim_ode(ode=mod_BA_HLT,
regimen=reg_mod_BA,
parameters=par_mod_BA)

But when I try to run the simulation is says:
Error in sim(...) :
Not all covariates for this model have been specified. Missing:
HLT

but I don't understand because I specified it in my opinion. I tried to fix it but I can't figure out what I am doing wrong...

Thank you so much for any help!:)