Test for constant expressions in Symbolics.jl

New to Julia
5

I tried the following code -

#!/snap/bin/julia

using Pkg
Pkg.activate("symbolics", shared = true)
using Symbolics
using SymbolicUtils

@variables x,y,z

expr = x*sin(y)*z^2

println(SymbolicUtils.istree(expr))

and the answer was false. Am I doing something wrong here because if istree is true I can use the arguments function to see if a particular arg is in the tree.