diff --git a/docs/changelog.md b/docs/changelog.md
index 68877f7..8540d50 100644
--- a/docs/changelog.md
+++ b/docs/changelog.md
@@ -6,6 +6,9 @@
  #42](https://github.com/pelme/htpy/issues/42).
  - Run tests on Python 3.13 RC (no changes were required, earlier versions
  should work fine too). [PR #45](https://github.com/pelme/htpy/pull/45).
+ - Attributes that are not strings will now be rejected runtime. Attributes have
+ been typed as strings previously but this is now also enforced during runtime.
+ If you need to pass non-strings as attribute values, wrap them in str() calls.
 
 ## 24.8.0 - 2024-08-03
 - Allow conditional rendering based on `bool`. [PR #40](https://github.com/pelme/htpy/pull/41).
diff --git a/htpy/__init__.py b/htpy/__init__.py
index 3c03930..4689f72 100644
--- a/htpy/__init__.py
+++ b/htpy/__init__.py
@@ -80,7 +80,10 @@ def _kwarg_attribute_name(name: str) -> str:
 
 def _generate_attrs(raw_attrs: dict[str, Attribute]) -> Iterable[tuple[str, Attribute]]:
     for key, value in raw_attrs.items():
-        if value in (False, None):
+        if not isinstance(key, str):  # pyright: ignore [reportUnnecessaryIsInstance]
+            raise ValueError("Attribute key must be a string")
+
+        if value is False or value is None:
             continue
 
         if key == "class":
@@ -91,6 +94,9 @@ def _generate_attrs(raw_attrs: dict[str, Attribute]) -> Iterable[tuple[str, Attr
             yield _force_escape(key), True
 
         else:
+            if not isinstance(value, str | _HasHtml):
+                raise ValueError(f"Attribute value must be a string , got {value!r}")
+
             yield _force_escape(key), _force_escape(value)
 
 
@@ -118,9 +124,9 @@ def iter_node(x: Node) -> Iterator[str]:
 
     if isinstance(x, BaseElement):
         yield from x
-    elif isinstance(x, str) or hasattr(x, "__html__"):
+    elif isinstance(x, str | _HasHtml):
         yield str(_escape(x))
-    elif isinstance(x, Iterable):
+    elif isinstance(x, Iterable):  # pyright: ignore [reportUnnecessaryIsInstance]
         for child in x:
             yield from iter_node(child)
     else:
@@ -234,6 +240,7 @@ def comment(text: str) -> _Markup:
     return _Markup(f"<!-- {escaped_text} -->")
 
 
+@t.runtime_checkable
 class _HasHtml(t.Protocol):
     def __html__(self) -> str: ...
 
diff --git a/tests/test_attributes.py b/tests/test_attributes.py
index 6d3975c..13c09eb 100644
--- a/tests/test_attributes.py
+++ b/tests/test_attributes.py
@@ -1,3 +1,5 @@
+import typing as t
+
 import pytest
 from markupsafe import Markup
 
@@ -177,3 +179,18 @@ def test_class_priority() -> None:
 def test_attribute_priority() -> None:
     result = div({"foo": "a"}, foo="b")
     assert str(result) == """<div foo="b"></div>"""
+
+
+@pytest.mark.parametrize("not_an_attr", [1234, b"foo", object(), object, 1, 0, None])
+def test_invalid_attribute_key(not_an_attr: t.Any) -> None:
+    with pytest.raises(ValueError, match="Attribute key must be a string"):
+        str(div({not_an_attr: "foo"}))
+
+
+@pytest.mark.parametrize(
+    "not_an_attr",
+    [1234, b"foo", object(), object, 1, 0],
+)
+def test_invalid_attribute_value(not_an_attr: t.Any) -> None:
+    with pytest.raises(ValueError, match="Attribute value must be a string"):
+        str(div(foo=not_an_attr))