diff --git a/src/reactions.jl b/src/reactions.jl
index 86e3409..1bed3cb 100644
--- a/src/reactions.jl
+++ b/src/reactions.jl
@@ -163,7 +163,7 @@ Get rate constant of mass action kineticLaws
 function get_massaction(kl::Num, reactants::Union{Vector{Num}, Nothing},
         stoich::Union{Vector{<:Real}, Nothing})
     function check_args(x::SymbolicUtils.BasicSymbolic{Real})
-        check_args(Val(SymbolicUtils.istree(x)), x)
+        check_args(Val(SymbolicUtils.iscall(x)), x)
     end
 
     function check_args(::Val{true}, x::SymbolicUtils.BasicSymbolic{Real})