Hi, It's much easier for us to help if you provide your example as a reproducible example, called a reprex .

After adding the missing `*`

s to the differential equations (see below) it ran without errors for me, with all 4 lines plotting.

What version of `dSolve`

are you running?

```
SEIR.dyn <- function(t, var, par) {
#variables and parameters
S <- var[1]
E <- var[2]
I <- var[3]
R <- var[4]
N <- S + E + I + R
beta <- par[1]
gamma <- par[2]
delta <- par[3]
#differential equations
dS <- (-beta*S*I)
dE <- (beta*S*I) - (delta*E)
dI <- (delta*E) - (gamma*I)
dR <- (gamma*I)
list(c(dS, dE, dI, dR))
}
```