diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index a7bba41..a9a0913 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -93,7 +93,7 @@ jobs:
       - name: Install dependencies
         run: |
           pip install -r requirements.txt
-          pip install black isort flake8
+          pip install black==23.12.1 isort flake8
 
       - name: Compile Cython extensions
         run: |
diff --git a/blacksheep/server/openapi/v3.py b/blacksheep/server/openapi/v3.py
index 0e191fe..416984b 100644
--- a/blacksheep/server/openapi/v3.py
+++ b/blacksheep/server/openapi/v3.py
@@ -779,7 +779,7 @@ def _try_get_schema_for_mapping(
             type=ValueType.OBJECT,
             properties={
                 properties_regexp.get(
-                    key_type, "^[a-zA-Z0-9_]+$"
+                    key_type, properties_regexp[str]
                 ): self.get_schema_by_type(value_type, context_type_args)
             },
         )