diff --git a/.github/workflows/cicd.yml b/.github/workflows/cicd.yml
index c799228b3..b9ea4cea9 100644
--- a/.github/workflows/cicd.yml
+++ b/.github/workflows/cicd.yml
@@ -77,6 +77,9 @@ jobs:
     - name: Test with pytest
       run: |
         poetry run pytest --cov=./ --cov-report xml
+        if [[ ${{ matrix.python-version }} == '3.9' ]]; then
+          poetry run pytest --cov=./ --cov-report xml --cov-append -m slow
+        fi
     - name: Upload coverage to Codecov
       uses: codecov/codecov-action@81cd2dc8148241f03f5839d295e000b8f761e378  # v3.1.0
     - name: Save cache
diff --git a/annif/openapi/annif.yaml b/annif/openapi/annif.yaml
index 117faef9c..09d26a2a0 100644
--- a/annif/openapi/annif.yaml
+++ b/annif/openapi/annif.yaml
@@ -224,6 +224,7 @@ components:
         notation:
           type: string
           example: "42.42"
+          nullable: true
         score:
           type: number
           example: 0.85
@@ -246,6 +247,8 @@ components:
           type: array
           items:
             type: object
+            required:
+              - uri
             properties:
               uri:
                 type: string
diff --git a/pyproject.toml b/pyproject.toml
index 4c7fb93a9..5a69bc4b1 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -91,3 +91,6 @@ build-backend = "poetry.core.masonry.api"
 profile = "black"
 line_length = "88"
 skip_gitignore = true
+
+[tool.pytest.ini_options]
+addopts = "-m 'not slow'"
diff --git a/tests/test_openapi.py b/tests/test_openapi.py
index e77230fd9..91de50385 100644
--- a/tests/test_openapi.py
+++ b/tests/test_openapi.py
@@ -1,4 +1,5 @@
 """Unit tests for Annif REST API / Swagger spec"""
+import pytest
 import schemathesis
 from hypothesis import settings
 
@@ -15,3 +16,12 @@ def check_cors(response, case):
 def test_api(case, app):
     response = case.call_wsgi(app)
     case.validate_response(response, additional_checks=(check_cors,))
+
+
+@pytest.mark.slow
+@schema.parametrize(endpoint="/v1/projects/{project_id}")
+@settings(max_examples=50)
+def test_api_target_dummy_fi(case, app):
+    case.path_parameters = {"project_id": "dummy-fi"}
+    response = case.call_wsgi(app)
+    case.validate_response(response)