diff --git a/pyiceberg/avro/resolver.py b/pyiceberg/avro/resolver.py
index 3f19ae9878..430e3bb171 100644
--- a/pyiceberg/avro/resolver.py
+++ b/pyiceberg/avro/resolver.py
@@ -147,7 +147,7 @@ def struct(self, struct: StructType, field_results: list[Writer]) -> Writer:
         return StructWriter(tuple((pos, result) for pos, result in enumerate(field_results)))
 
     def field(self, field: NestedField, field_result: Writer) -> Writer:
-        return field_result if field.required else OptionWriter(field_result)
+        return field_result if field.required or isinstance(field.field_type, UnknownType) else OptionWriter(field_result)
 
     def list(self, list_type: ListType, element_result: Writer) -> Writer:
         return ListWriter(element_result)
@@ -302,7 +302,9 @@ def struct(self, file_schema: StructType, record_struct: IcebergType | None, fil
         return StructWriter(field_writers=tuple(results))
 
     def field(self, file_field: NestedField, record_type: IcebergType | None, field_writer: Writer) -> Writer:
-        return field_writer if file_field.required else OptionWriter(field_writer)
+        return (
+            field_writer if file_field.required or isinstance(file_field.field_type, UnknownType) else OptionWriter(field_writer)
+        )
 
     def list(self, file_list_type: ListType, file_list: IcebergType | None, element_writer: Writer) -> Writer:
         return ListWriter(element_writer if file_list_type.element_required else OptionWriter(element_writer))
diff --git a/pyiceberg/utils/schema_conversion.py b/pyiceberg/utils/schema_conversion.py
index 7e6343141b..f6448aa565 100644
--- a/pyiceberg/utils/schema_conversion.py
+++ b/pyiceberg/utils/schema_conversion.py
@@ -534,7 +534,8 @@ def field(self, field: NestedField, field_result: AvroType) -> AvroType:
         result = {
             "name": sanitized_name,
             FIELD_ID_PROP: field.field_id,
-            "type": field_result if field.required else ["null", field_result],
+            # Unknown maps to Avro "null", and a union cannot contain "null" twice
+            "type": field_result if field.required or isinstance(field.field_type, UnknownType) else ["null", field_result],
         }
 
         if original_name != sanitized_name:
diff --git a/tests/avro/test_file.py b/tests/avro/test_file.py
index 175089c30b..ad2185d172 100644
--- a/tests/avro/test_file.py
+++ b/tests/avro/test_file.py
@@ -54,6 +54,7 @@
     TimestampType,
     TimestamptzType,
     TimeType,
+    UnknownType,
     UUIDType,
 )
 from pyiceberg.utils.schema_conversion import AvroSchemaConversion
@@ -544,3 +545,23 @@ def test_avro_output_file_tell_includes_buffered_records() -> None:
             out.write_block([manifest_entry(0)])
 
             assert out.tell() > after_header
+
+
+def test_write_optional_unknown_type() -> None:
+    # A partition field whose source column was dropped has the unknown type
+    schema = Schema(
+        NestedField(field_id=1, name="before", field_type=StringType(), required=False),
+        NestedField(field_id=2, name="dropped", field_type=UnknownType(), required=False),
+        NestedField(field_id=3, name="after", field_type=LongType(), required=True),
+    )
+
+    with TemporaryDirectory() as tmpdir:
+        tmp_avro_file = tmpdir + "/unknown.avro"
+        with avro.AvroOutputFile[Record](PyArrowFileIO().new_output(tmp_avro_file), schema, "unknown") as out:
+            out.write_block([Record("hello", None, 42)])
+
+        with avro.AvroFile[Record](PyArrowFileIO().new_input(tmp_avro_file), schema, {-1: Record}) as avro_reader:
+            assert list(avro_reader) == [Record("hello", None, 42)]
+
+        with open(tmp_avro_file, "rb") as fo:
+            assert list(reader(fo=fo)) == [{"before": "hello", "dropped": None, "after": 42}]
diff --git a/tests/catalog/test_catalog_behaviors.py b/tests/catalog/test_catalog_behaviors.py
index 4310ed1eb6..36efcb8cfb 100644
--- a/tests/catalog/test_catalog_behaviors.py
+++ b/tests/catalog/test_catalog_behaviors.py
@@ -848,6 +848,28 @@ def test_write_and_evolve(catalog: Catalog, format_version: int) -> None:
         txn.delete("foo = 'a'")
 
 
+@pytest.mark.parametrize("format_version", [1, 2])
+def test_delete_after_dropping_old_partition_source_column(catalog: Catalog, format_version: int) -> None:
+    identifier = f"default.delete_after_dropping_old_partition_source_v{format_version}"
+    catalog.create_namespace_if_not_exists("default")
+
+    pa_table = pa.Table.from_pydict({"id": [1, 2], "category": ["a", "b"]})
+    tbl = catalog.create_table(identifier=identifier, schema=pa_table.schema, properties={"format-version": str(format_version)})
+    with tbl.update_spec() as spec_update:
+        spec_update.add_identity("category")
+    tbl.append(pa_table)
+
+    with tbl.update_spec() as spec_update:
+        spec_update.remove_field("category")
+    with tbl.update_schema() as schema_update:
+        schema_update.delete_column("category")
+
+    # Rewrites the manifest of the old spec, whose partition source column is gone
+    tbl.delete("id == 1")
+
+    assert catalog.load_table(identifier).scan().to_arrow()["id"].to_pylist() == [2]
+
+
 # Merge manifests
 @pytest.mark.parametrize("format_version", [1, 2])
 def test_merge_manifests_local_file_system(catalog: Catalog, arrow_table_with_null: pa.Table, format_version: int) -> None: