diff --git a/src/convert.jl b/src/convert.jl
index 498293a5..21b8ac4e 100644
--- a/src/convert.jl
+++ b/src/convert.jl
@@ -64,6 +64,7 @@ Other priorities are reserved for internal use.
 function pyconvert_add_rule(pytypename::String, type::Type, func::Function, priority::PyConvertPriority=PYCONVERT_PRIORITY_NORMAL)
     @nospecialize type func
     push!(get!(Vector{PyConvertRule}, PYCONVERT_RULES, pytypename), PyConvertRule(type, func, priority))
+    empty!.(values(PYCONVERT_RULES_CACHE))
     return
 end
 
diff --git a/test/convert.jl b/test/convert.jl
index 0051bf17..10e871d5 100644
--- a/test/convert.jl
+++ b/test/convert.jl
@@ -222,3 +222,19 @@ end
     x1 = pyconvert(DateTime, pydatetime(2001, 2, 3, 4, 5, 6, 7000))
     @test x1 === DateTime(2001, 2, 3, 4, 5, 6, 7)
 end
+
+@testitem "pyconvert_add_rule (#364)" begin
+    id = string(rand(UInt128), base=16)
+    pyexec("""
+    class Hello_364_$id:
+        pass
+    """, @__MODULE__)
+    x = pyeval("Hello_364_$id()", @__MODULE__)
+    @test pyconvert(Any, x) === x # This test has a side effect of influencing the rules cache
+    t = pytype(x)
+    PythonCall.pyconvert_add_rule("$(t.__module__):$(t.__qualname__)", String, (_, _) -> "Hello!!")
+    @test pyconvert(String, x) == "Hello!!"
+    @test pyconvert(Any, x) == "Hello!!" # Broken before PR #365
+end
+
+end