diff --git a/Project.toml b/Project.toml
index a26221388..274421ecb 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,7 +1,7 @@
 name = "Mooncake"
 uuid = "da2b9cff-9c12-43a0-ae48-6db2b0edb7d6"
 authors = ["Will Tebbutt, Hong Ge, and contributors"]
-version = "0.4.33"
+version = "0.4.34"
 
 [deps]
 ADTypes = "47edcb42-4c32-4615-8424-f2b9edc5f35b"
diff --git a/src/interpreter/s2s_reverse_mode_ad.jl b/src/interpreter/s2s_reverse_mode_ad.jl
index e01c62195..d93f3fa3e 100644
--- a/src/interpreter/s2s_reverse_mode_ad.jl
+++ b/src/interpreter/s2s_reverse_mode_ad.jl
@@ -794,6 +794,8 @@ end
 _is_primitive(C::Type, mi::Core.MethodInstance) = is_primitive(C, mi.specTypes)
 _is_primitive(C::Type, sig::Type) = is_primitive(C, sig)
 
+const RuleMC{A, R} = MistyClosure{OpaqueClosure{A, R}}
+
 # Compute the concrete type of the rule that will be returned from `build_rrule`. This is
 # important for performance in dynamic dispatch, and to ensure that recursion works
 # properly.
@@ -808,20 +810,30 @@ function rule_type(interp::MooncakeInterpreter{C}, sig_or_mi; debug_mode) where
     isva, _ = is_vararg_and_sparam_names(sig_or_mi)
 
     arg_types = map(_type, ir.argtypes)
-    primal_sig = Tuple{arg_types...}
+    sig = Tuple{arg_types...}
     arg_fwds_types = Tuple{map(fcodual_type, arg_types)...}
     arg_rvs_types = Tuple{map(rdata_type ∘ tangent_type, arg_types)...}
     rvs_return_type = rdata_type(tangent_type(Treturn))
-    pb_type = MistyClosure{OpaqueClosure{Tuple{rvs_return_type}, arg_rvs_types}}
-
-    Tderived_rule = DerivedRule{
-        primal_sig,
-        MistyClosure{OpaqueClosure{arg_fwds_types, fcodual_type(Treturn)}},
-        Pullback{primal_sig, Base.RefValue{pb_type}, Val{isva}, nvargs(isva, primal_sig)},
-        Val{isva},
-        Val{length(ir.argtypes)},
-    }
-    return debug_mode ? DebugRRule{Tderived_rule} : Tderived_rule
+    pb_oc_type = MistyClosure{OpaqueClosure{Tuple{rvs_return_type}, arg_rvs_types}}
+    pb_type = Pullback{sig, Base.RefValue{pb_oc_type}, Val{isva}, nvargs(isva, sig)}
+    nargs = Val{length(ir.argtypes)}
+
+    if isconcretetype(Treturn)
+        Tderived_rule = DerivedRule{
+            sig, RuleMC{arg_fwds_types, fcodual_type(Treturn)}, pb_type, Val{isva}, nargs,
+        }
+        return debug_mode ? DebugRRule{Tderived_rule} : Tderived_rule
+    else
+        if debug_mode
+            return DebugRRule{DerivedRule{
+                sig, RuleMC{arg_fwds_types, P}, pb_type, Val{isva}, nargs,
+            }} where {P<:fcodual_type(Treturn)}
+        else
+            return DerivedRule{
+                sig, RuleMC{arg_fwds_types, P}, pb_type, Val{isva}, nargs,
+            } where {P<:fcodual_type(Treturn)}
+        end
+    end
 end
 
 nvargs(isva, sig) = Val{isva ? length(sig.parameters[end].parameters) : 0}
diff --git a/src/test_resources.jl b/src/test_resources.jl
index 387410c28..5075646fb 100644
--- a/src/test_resources.jl
+++ b/src/test_resources.jl
@@ -577,6 +577,8 @@ function typevar_tester()
     return UnionAll(tv, t)
 end
 
+tuple_with_union(x::Bool) = (x ? 5.0 : 5, nothing)
+
 function generate_test_functions()
     return Any[
         (false, :allocs, nothing, const_tester),
diff --git a/test/interpreter/s2s_reverse_mode_ad.jl b/test/interpreter/s2s_reverse_mode_ad.jl
index 15988cc17..097119a40 100644
--- a/test/interpreter/s2s_reverse_mode_ad.jl
+++ b/test/interpreter/s2s_reverse_mode_ad.jl
@@ -244,8 +244,9 @@ end
     @testset "rule_type $sig, $debug_mode" for
         sig in Any[
             Tuple{typeof(getfield), Tuple{Float64}, 1},
-            Tuple{typeof(Mooncake.TestResources.foo), Float64},
-            Tuple{typeof(Mooncake.TestResources.type_unstable_tester_0), Ref{Any}},
+            Tuple{typeof(TestResources.foo), Float64},
+            Tuple{typeof(TestResources.type_unstable_tester_0), Ref{Any}},
+            Tuple{typeof(TestResources.tuple_with_union), Bool},
         ],
         debug_mode in [true, false]