From yaml null index

pandera/io.py

@@ -156,7 +156,9 @@ def _deserialize_schema(serialized_schema):
             for index_component in serialized_schema["index"]
         ]
 
-    if len(index) == 1:
+    if index is None:
+        index = None
+    elif len(index) == 1:
         index = Index(**index[0])
     else:
         index = MultiIndex(indexes=[

tests/test_io.py

@@ -125,6 +125,63 @@ def _create_schema(index="single"):
 """.format(version=pa.__version__)
 
 
+def _create_schema_null_index():
+
+    return pa.DataFrameSchema(
+        columns={
+            "float_column": pa.Column(
+                pa.Float, checks=[
+                    pa.Check.greater_than(-10),
+                    pa.Check.less_than(20),
+                    pa.Check.in_range(-10, 20),
+                ]
+            ),
+            "str_column": pa.Column(
+                pa.String, checks=[
+                    pa.Check.isin(["foo", "bar", "x", "xy"]),
+                    pa.Check.str_length(1, 3)
+                ]
+            ),
+        },
+        index=None
+    )
+
+
+YAML_SCHEMA_NULL_INDEX = """
+schema_type: dataframe
+version: {version}
+columns:
+  float_column:
+    pandas_dtype: float
+    nullable: false
+    checks:
+      greater_than: -10
+      less_than: 20
+      in_range:
+        min_value: -10
+        max_value: 20
+  str_column:
+    pandas_dtype: string
+    nullable: false
+    checks:
+      isin:
+      - foo
+      - bar
+      - x
+      - xy
+      str_length:
+        min_value: 1
+        max_value: 3
+index: null
+coerce: false
+strict: false
+""".format(version=pa.__version__)
+
+YAML_VALIDATION_PAIRS = [
+    [YAML_SCHEMA, _create_schema],
+    [YAML_SCHEMA_NULL_INDEX, _create_schema_null_index]
+]
+
 @pytest.mark.skipif(
     PYYAML_VERSION.release < (5, 1, 0),  # type: ignore
     reason="pyyaml >= 5.1.0 required",
@@ -162,10 +219,12 @@ def test_to_yaml():
 )
 def test_from_yaml():
     """Test that from_yaml reads yaml string."""
-    schema_from_yaml = io.from_yaml(YAML_SCHEMA)
-    expected_schema = _create_schema()
-    assert schema_from_yaml == expected_schema
-    assert expected_schema == schema_from_yaml
+
+    for yml_string, schema_creator in YAML_VALIDATION_PAIRS:
+        schema_from_yaml = io.from_yaml(yml_string)
+        expected_schema = schema_creator()
+        assert schema_from_yaml == expected_schema
+        assert expected_schema == schema_from_yaml
 
 
 def test_io_yaml_file_obj():