Update test/test_util.jl

Co-Authored-By: Lyndon White <oxinabox@ucc.asn.au>

Author: simeonschaub

test/test_util.jl

@@ -162,7 +162,7 @@ end
 function Base.isapprox(d_ad::DNE, d_fd; kwargs...)
     error("Tried to differentiate w.r.t. a DNE")
 end
-function Base.isapprox(d_ad::Union{Thunk,One,Zero}, d_fd; kwargs...)
+function Base.isapprox(d_ad::AbstractDifferential, d_fd; kwargs...)
     return isapprox(extern(d_ad), d_fd; kwargs...)
 end