diff --git a/Project.toml b/Project.toml
index 3c5b0db0cb..59334e079e 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 = "0.4.150"
+version = "0.4.151"
 
 [deps]
 ADTypes = "47edcb42-4c32-4615-8424-f2b9edc5f35b"
diff --git a/src/interpreter/reverse_mode.jl b/src/interpreter/reverse_mode.jl
index af989bc248..bb7fb941f6 100644
--- a/src/interpreter/reverse_mode.jl
+++ b/src/interpreter/reverse_mode.jl
@@ -1746,7 +1746,14 @@ DynamicDerivedRule(debug_mode::Bool) = DynamicDerivedRule(Dict{Any,Any}(), debug
 _copy(x::P) where {P<:DynamicDerivedRule} = P(Dict{Any,Any}(), x.debug_mode)
 
 function (dynamic_rule::DynamicDerivedRule)(args::Vararg{Any,N}) where {N}
-    sig = Tuple{map(_typeof ∘ primal, args)...}
+
+    # `Base._stable_typeof` is used here, rather than `typeof` or `Mooncake._typeof`. Its
+    # precise behaviour (equivalent to `typeof` for everything except `Type`s, for which it
+    # returns `Type{P}` rather than `typeof(P)`) is needed to ensure that this signature
+    # matches the types that `rule` sees when `rule(args...)` is called below. If you get
+    # this wrong, an assertion is violated, causing a hard-to-debug error (see issue 660).
+    sig = Tuple{map(Base._stable_typeof ∘ primal, args)...}
+
     rule = get(dynamic_rule.cache, sig, nothing)
     if rule === nothing
         interp = get_interpreter(ReverseMode)
diff --git a/test/interpreter/reverse_mode.jl b/test/interpreter/reverse_mode.jl
index 8b930f2a4b..b1986ffdab 100644
--- a/test/interpreter/reverse_mode.jl
+++ b/test/interpreter/reverse_mode.jl
@@ -14,6 +14,9 @@ f(a, x) = dot(a.data, x)
 
 unstable_tester(x::Ref{Any}) = sin(x[])
 
+# used for regression test for issue 660
+struct MakeAUnionAll{T} end
+
 end
 
 @testset "s2s_reverse_mode_ad" begin
@@ -353,6 +356,11 @@ end
         f(x) = sin(cos(x))
         rule = Mooncake.build_rrule(f, 0.0)
         @benchmark Mooncake.value_and_gradient!!($rule, $f, $(Ref(0.0))[])
+
+        # 660 -- ensure that the correct signature is used to construct DynamicDerivedRules
+        rule = Mooncake.DynamicDerivedRule(false)
+        args = (zero_fcodual(identity), zero_fcodual((v=S2SGlobals.MakeAUnionAll,)))
+        @test rule(args...) isa Tuple{CoDual,Any}
     end
     @testset "literal Strings do not appear in shared data" begin
         f() = "hello"