diff --git a/requirements-dev.txt b/requirements-dev.txt
index 399f9b7417335..c478ddd21f536 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -21,6 +21,7 @@ flask-testing==0.8.0
 ipdb==0.12
 isort==4.3.21
 mypy==0.770
+openapi-spec-validator==0.2.8
 pytest==5.4.3
 pytest-cov==2.10.0
 parameterized==0.7.4
diff --git a/superset/charts/api.py b/superset/charts/api.py
index 6c8635bbeb5ac..4901041d279c2 100644
--- a/superset/charts/api.py
+++ b/superset/charts/api.py
@@ -645,7 +645,7 @@ def thumbnail(
           responses:
             200:
               description: Chart thumbnail image
-              /content:
+              content:
                image/*:
                  schema:
                    type: string
diff --git a/superset/charts/schemas.py b/superset/charts/schemas.py
index 385640439137e..0449f65da2c0f 100644
--- a/superset/charts/schemas.py
+++ b/superset/charts/schemas.py
@@ -31,13 +31,12 @@
 
 width_height_schema = {
     "type": "array",
-    "items": [{"type": "integer"}, {"type": "integer"}],
+    "items": {"type": "integer"},
 }
 thumbnail_query_schema = {
     "type": "object",
     "properties": {"force": {"type": "boolean"}},
 }
-
 screenshot_query_schema = {
     "type": "object",
     "properties": {
diff --git a/tests/base_api_tests.py b/tests/base_api_tests.py
index ba7dde5cbea91..2be642ce4f02d 100644
--- a/tests/base_api_tests.py
+++ b/tests/base_api_tests.py
@@ -48,6 +48,22 @@ class Model1Api(BaseSupersetModelRestApi):
 appbuilder.add_api(Model1Api)
 
 
+class TestOpenApiSpec(SupersetTestCase):
+    def test_open_api_spec(self):
+        """
+        API: Test validate OpenAPI spec
+        :return:
+        """
+        from openapi_spec_validator import validate_spec
+
+        self.login(username="admin")
+        uri = "api/v1/_openapi"
+        rv = self.client.get(uri)
+        self.assertEqual(rv.status_code, 200)
+        response = json.loads(rv.data.decode("utf-8"))
+        validate_spec(response)
+
+
 class TestBaseModelRestApi(SupersetTestCase):
     def test_default_missing_declaration_get(self):
         """