diff --git a/tests/test_core.py b/tests/test_core.py
index 688eca6..5261e9b 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py
@@ -222,29 +222,42 @@ def test_dataset_empty_constructor():
 
 def test_dataset_example(ds):
     ds_schema = DatasetSchema(
-        {
+        data_vars={
             'foo': DataArraySchema(name='foo', dtype=np.int32, dims=['x']),
             'bar': DataArraySchema(name='bar', dtype=np.floating, dims=['x', 'y']),
-        }
+        },
+        coords={'x': DataArraySchema(name='x', dtype=np.int64, dims=['x'])},
+        attrs={},
     )
 
     jsonschema.validate(ds_schema.json, ds_schema._json_schema)
 
     assert list(ds_schema.json['data_vars'].keys()) == ['foo', 'bar']
+    assert list(ds_schema.json['coords']['coords'].keys()) == ['x']
     ds_schema.validate(ds)
 
-    ds['foo'] = ds.foo.astype('float32')
+    ds2 = ds.copy()
+    ds2['foo'] = ds2.foo.astype('float32')
     with pytest.raises(SchemaError, match='dtype'):
-        ds_schema.validate(ds)
+        ds_schema.validate(ds2)
 
-    ds = ds.drop_vars('foo')
+    ds2 = ds2.drop_vars('foo')
     with pytest.raises(SchemaError, match='variable foo'):
-        ds_schema.validate(ds)
+        ds_schema.validate(ds2)
+
+    ds3 = ds.copy()
+    ds3['x'] = ds3.x.astype('float32')
+    with pytest.raises(SchemaError, match='dtype'):
+        ds_schema.validate(ds3)
+
+    ds3 = ds3.drop_vars('x')
+    with pytest.raises(SchemaError, match='coords has missing keys'):
+        ds_schema.validate(ds3)
 
     # json roundtrip
     rt_schema = DatasetSchema.from_json(ds_schema.json)
     assert isinstance(rt_schema, DatasetSchema)
-    rt_schema.json == ds_schema.json
+    assert rt_schema.json == ds_schema.json
 
 
 def test_checks_ds(ds):
diff --git a/xarray_schema/dataset.py b/xarray_schema/dataset.py
index 9be35ee..a7d2289 100644
--- a/xarray_schema/dataset.py
+++ b/xarray_schema/dataset.py
@@ -47,9 +47,9 @@ def from_json(cls, obj: dict):
                 k: DataArraySchema.from_json(v) for k, v in obj['data_vars'].items()
             }
         if 'coords' in obj:
-            kwargs['coords'] = {k: CoordsSchema.from_json(v) for k, v in obj['coords'].items()}
+            kwargs['coords'] = CoordsSchema.from_json(obj['coords'])
         if 'attrs' in obj:
-            kwargs['attrs'] = {k: AttrsSchema.from_json(v) for k, v in obj['attrs'].items()}
+            kwargs['attrs'] = AttrsSchema.from_json(obj['attrs'])
 
         return cls(**kwargs)
 
@@ -79,8 +79,8 @@ def validate(self, ds: xr.Dataset) -> None:
                     else:
                         da_schema.validate(ds.data_vars[key])
 
-        if self.coords is not None:  # pragma: no cover
-            raise NotImplementedError('coords schema not implemented yet')
+        if self.coords is not None:
+            self.coords.validate(ds.coords)
 
         if self.attrs:
             self.attrs.validate(ds.attrs)