diff --git a/AUTHORS.rst b/AUTHORS.rst
index 309c1916..e10eb08f 100644
--- a/AUTHORS.rst
+++ b/AUTHORS.rst
@@ -64,3 +64,4 @@ Contributors (chronological)
 - Fedor Fominykh `@fedorfo <https://github.com/fedorfo>`_
 - Colin Bounouar `@Colin-b <https://github.com/Colin-b>`_
 - David Bishop `@teancom <https://github.com/teancom>`_
+- Andrea Ghensi `@sanzoghenzo <https://github.com/sanzoghenzo>`_
diff --git a/src/apispec/ext/marshmallow/field_converter.py b/src/apispec/ext/marshmallow/field_converter.py
index 8130ef0b..07a72559 100644
--- a/src/apispec/ext/marshmallow/field_converter.py
+++ b/src/apispec/ext/marshmallow/field_converter.py
@@ -397,6 +397,7 @@ def metadata2properties(self, field, **kwargs):
         metadata = {
             key.replace("_", "-") if key.startswith("x_") else key: value
             for key, value in field.metadata.items()
+            if isinstance(key, str)
         }
 
         # Avoid validation error with "Additional properties not allowed"
diff --git a/tests/test_ext_marshmallow_field.py b/tests/test_ext_marshmallow_field.py
index 701da30d..274f0d9f 100644
--- a/tests/test_ext_marshmallow_field.py
+++ b/tests/test_ext_marshmallow_field.py
@@ -293,3 +293,13 @@ def custom_string2properties(self, field, **kwargs):
     assert properties["x-customString"] == (
         spec_fixture.openapi.openapi_version == "2.0"
     )
+
+
+def test_field2property_with_non_string_metadata_keys(spec_fixture):
+    class _DesertSentinel:
+        pass
+
+    field = fields.Boolean(description="A description")
+    field.metadata[_DesertSentinel()] = "to be ignored"
+    result = spec_fixture.openapi.field2property(field)
+    assert result == {"description": "A description", "type": "boolean"}