diff --git a/src/eval.jl b/src/eval.jl
index d66ac55..68eadd1 100644
--- a/src/eval.jl
+++ b/src/eval.jl
@@ -48,15 +48,18 @@ import ChainRulesCore
 
 function ChainRulesCore.rrule(ev::Eval, args...)
     Z = ev.fwd(args...)
-    Z, function tullio_back(Δ)
-        isnothing(ev.rev) && error("no gradient definition here!")
+    function tullio_back(Δ)
         dxs = map(ev.rev(Δ, Z, args...)) do dx
             dx === nothing ? ChainRulesCore.ZeroTangent() : dx
         end
-        tuple(ChainRulesCore.ZeroTangent(), dxs...)
+        return (ChainRulesCore.NoTangent(), dxs...)
     end
+    return Z, tullio_back
 end
 
+# without gradient definition we let the AD system differentiate the function
+ChainRulesCore.@opt_out ChainRulesCore.rrule(ev::Eval{<:Any,Nothing}, args...)
+
 @init @require FillArrays = "1a297f60-69ca-5386-bcde-b61e274b549b" begin
     using .FillArrays: Fill # used by Zygote
     Tullio.promote_storage(::Type{T}, ::Type{F}) where {T, F<:Fill} = T