diff --git a/HISTORY.md b/HISTORY.md
index f81acc7a..f2decefa 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -13,6 +13,8 @@ Our backwards-compatibility policy can be found [here](https://github.com/python
 
 ## NEXT (UNRELEASED)
 
+- Fix `ForbiddenExtraKeysError` and {func}`transform_error <cattrs.transform_error>` crashing with `TypeError` when an extra key isn't a string, like an int key from YAML.
+  ([#784](https://github.com/python-attrs/cattrs/pull/784))
 - Fix heterogeneous tuples and `NamedTuple`s with a member type containing a quote in its `repr`, like `tuple[Literal["a"], int]`, crashing structuring code generation with `SyntaxError`; the index note is now embedded with `repr`.
   ([#777](https://github.com/python-attrs/cattrs/pull/777))
 - Fix {func}`transform_error <cattrs.transform_error>` listing the extra keys of a `ForbiddenExtraKeysError` in set iteration order, which made the message differ between runs; the keys are now sorted, like the error's own `__str__` already sorts them.
diff --git a/src/cattrs/errors.py b/src/cattrs/errors.py
index 13df1009..f8db816f 100644
--- a/src/cattrs/errors.py
+++ b/src/cattrs/errors.py
@@ -138,5 +138,5 @@ def __str__(self) -> str:
         return (
             self.message
             or f"Extra fields in constructor for {self.cl.__name__}: "
-            f"{', '.join(sorted(self.extra_fields))}"
+            f"{', '.join(sorted(map(str, self.extra_fields)))}"
         )
diff --git a/src/cattrs/v.py b/src/cattrs/v.py
index 78361aab..e089bdf7 100644
--- a/src/cattrs/v.py
+++ b/src/cattrs/v.py
@@ -41,7 +41,7 @@ def format_exception(exc: BaseException, type: Union[type, None]) -> str:
             tn = type.__name__ if hasattr(type, "__name__") else repr(type)
             res = f"invalid value for type, expected {tn}"
     elif isinstance(exc, ForbiddenExtraKeysError):
-        res = f"extra fields found ({', '.join(sorted(exc.extra_fields))})"
+        res = f"extra fields found ({', '.join(sorted(map(str, exc.extra_fields)))})"
     elif isinstance(exc, AttributeError) and exc.args[0].endswith(
         "object has no attribute 'items'"
     ):
diff --git a/tests/test_v.py b/tests/test_v.py
index 4456b60e..7f7021cb 100644
--- a/tests/test_v.py
+++ b/tests/test_v.py
@@ -127,6 +127,26 @@ class C:
     assert transform_error(exc_info.value) == ["extra fields found (b, c, d, e) @ $"]
 
 
+def test_extra_keys_not_strings(c: Converter) -> None:
+    """Extra keys that aren't strings, like ints from YAML, are formatted."""
+
+    @define
+    class C:
+        a: int
+
+    c.register_structure_hook(
+        C, make_dict_structure_fn(C, c, _cattrs_forbid_extra_keys=True)
+    )
+
+    with raises(Exception) as exc_info:
+        c.structure({"a": 1, 2: 2, "b": 3}, C)
+
+    assert transform_error(exc_info.value) == ["extra fields found (2, b) @ $"]
+    assert (
+        str(exc_info.value.exceptions[0]) == "Extra fields in constructor for C: 2, b"
+    )
+
+
 def test_untyped_class_errors(c: Converter) -> None:
     """Errors on untyped attrs classes transform correctly."""