diff --git a/include/pybind11/detail/internals.h b/include/pybind11/detail/internals.h
index 812505c22..75fcd3c20 100644
--- a/include/pybind11/detail/internals.h
+++ b/include/pybind11/detail/internals.h
@@ -266,7 +266,7 @@ PYBIND11_NOINLINE inline internals &get_internals() {
         const PyGILState_STATE state;
     } gil;
 
-    constexpr auto *id = PYBIND11_INTERNALS_ID;
+    PYBIND11_STR_TYPE id(PYBIND11_INTERNALS_ID);
     auto builtins = handle(PyEval_GetBuiltins());
     if (builtins.contains(id) && isinstance<capsule>(builtins[id])) {
         internals_pp = static_cast<internals **>(capsule(builtins[id]));
diff --git a/tests/test_modules.py b/tests/test_modules.py
index 5630ccf9b..3390031af 100644
--- a/tests/test_modules.py
+++ b/tests/test_modules.py
@@ -75,3 +75,16 @@ def test_duplicate_registration():
     """Registering two things with the same name"""
 
     assert m.duplicate_registration() == []
+
+
+def test_builtin_key_type():
+    """Test that all the keys in the builtin modules have type str.
+
+    Previous versions of pybind11 would add a unicode key in python 2.
+    """
+    if hasattr(__builtins__, "keys"):
+        keys = __builtins__.keys()
+    else:  # this is to make pypy happy since builtins is different there.
+        keys = __builtins__.__dict__.keys()
+
+    assert {type(k) for k in keys} == {str}