diff --git a/myst_parser/config/main.py b/myst_parser/config/main.py
index 24e2a540..4a2e646b 100644
--- a/myst_parser/config/main.py
+++ b/myst_parser/config/main.py
@@ -91,10 +91,9 @@ def check_url_schemes(inst: "MdParserConfig", field: dc.Field, value: Any) -> No
                 raise TypeError(
                     f"'{field.name}[{key}][title]' is not a string: {val['title']!r}"
                 )
-            if (
-                "classes" in val
-                and not isinstance(val["classes"], list)
-                and not all(isinstance(c, str) for c in val["classes"])
+            if "classes" in val and (
+                not isinstance(val["classes"], list)
+                or not all(isinstance(c, str) for c in val["classes"])
             ):
                 raise TypeError(
                     f"'{field.name}[{key}][classes]' is not a list of str: {val['classes']!r}"
diff --git a/tests/test_config_topmatter.py b/tests/test_config_topmatter.py
index 58be4461..af030a50 100644
--- a/tests/test_config_topmatter.py
+++ b/tests/test_config_topmatter.py
@@ -225,6 +225,29 @@ def test_the_dict_form_keeps_its_templates(self):
         assert warnings == []
         assert config.url_schemes == {"wiki": {"url": "https://w.org/{{path}}"}}
 
+    @pytest.mark.parametrize(
+        "classes",
+        ["my-class", ["a", 1], 1],
+        ids=["string", "non-string-item", "int"],
+    )
+    def test_classes_must_be_a_list_of_strings(self, classes):
+        """A bare string was accepted, and split into one class per character;
+        a list holding a non-string was accepted, and crashed the HTML writer.
+        """
+        with pytest.raises(TypeError, match=r"\[classes\]' is not a list of str"):
+            MdParserConfig(url_schemes={"gh": {"url": "{{path}}", "classes": classes}})
+        _config, warnings = merge(
+            {"myst": {"url_schemes": {"gh": {"url": "{{path}}", "classes": classes}}}}
+        )
+        assert len(warnings) == 1
+        assert "is not a list of str" in warnings[0]
+
+    def test_classes_list_of_strings_is_accepted(self):
+        config = MdParserConfig(
+            url_schemes={"gh": {"url": "{{path}}", "classes": ["a", "b"]}}
+        )
+        assert config.url_schemes["gh"]["classes"] == ["a", "b"]
+
 
 class TestGlobalOnlyWarningKind:
     """The global-only rejection is a ``myst.topmatter`` warning.