diff --git a/Makefile b/Makefile
index d8bd5be..543effc 100644
--- a/Makefile
+++ b/Makefile
@@ -86,7 +86,7 @@ lint:
 	$(PYTHON) -m flake8 --config=$(CURDIR)/tests/.flake8 tests
 	$(PYTHON) -m flake8 --config=$(CURDIR)/tests/.flake8 examples
 	$(PYTHON) -m isort --check-only --settings-path=$(CURDIR)/.isort.cfg src tests examples setup.py
-	MYPYPATH=src $(PYTHON) -m mypy --strict examples
+	MYPYPATH=src $(PYTHON) -m mypy --strict examples src
 	clang-format --Werror --dry-run src/cpp/*
 
 # https://www.npmjs.com/package/markdownlint-cli
diff --git a/news/6.feature.rst b/news/6.feature.rst
new file mode 100644
index 0000000..c735925
--- /dev/null
+++ b/news/6.feature.rst
@@ -0,0 +1 @@
+Enabled source code typechecking in the GitHub CI
diff --git a/src/blazingmq/_callbacks.py b/src/blazingmq/_callbacks.py
index c0b4129..9b39c82 100644
--- a/src/blazingmq/_callbacks.py
+++ b/src/blazingmq/_callbacks.py
@@ -21,6 +21,7 @@
 from typing import Callable
 from typing import Dict
 from typing import Iterable
+from typing import Mapping
 from typing import Optional
 from typing import TYPE_CHECKING
 from typing import Tuple
@@ -28,10 +29,11 @@
 from typing import Union
 import weakref
 
+from ._enums import AckStatus
 from ._enums import PropertyType
 from ._messages import Ack
-from ._messages import AckStatus
 from ._messages import Message
+from ._messages import MessageHandle
 from ._messages import create_ack
 from ._messages import create_message
 from ._messages import create_message_handle
@@ -90,7 +92,7 @@ def on_session_event(
             event_cls = failure_class_by_success_class[event_cls]
 
         assert queue_uri
-        event = event_cls(queue_uri, msg)
+        event: SessionEvent = event_cls(queue_uri, msg)
     else:
         event = event_cls(msg)
 
@@ -98,25 +100,24 @@ def on_session_event(
     user_callback(event)
 
 
-PropertiesAndTypesDictsType = Tuple[
-    Dict[str, Union[int, bytes]], Dict[str, PropertyType]
-]
+PropertiesAndTypesDictsType = Tuple[Dict[str, Union[int, bytes]], Dict[str, int]]
 
 
 def on_message(
-    user_callback: Callable[[Message], None],
+    user_callback: Callable[[Message, MessageHandle], None],
     ext_session_wr: weakref.ref[_ext.Session],
-    property_type_to_py: Dict[int, PropertyType],
+    property_type_to_py: Mapping[int, PropertyType],
     messages: Iterable[Tuple[bytes, bytes, bytes, PropertiesAndTypesDictsType]],
 ) -> None:
     ext_session = ext_session_wr()
     assert ext_session is not None, "ext.Session has been deleted"
     for data, guid, queue_uri, properties_tuple in messages:
         properties, property_types = properties_tuple
-        for k, v in property_types.items():
-            property_types[k] = property_type_to_py[v]
+        property_types_py = {
+            k: property_type_to_py[v] for k, v in property_types.items()
+        }
         message = create_message(
-            data, guid, queue_uri.decode(), properties, property_types
+            data, guid, queue_uri.decode(), properties, property_types_py
         )
         message_handle = create_message_handle(message, ext_session)
         user_callback(message, message_handle)
diff --git a/src/blazingmq/_script_name.py b/src/blazingmq/_script_name.py
index a9a88d3..0287d41 100644
--- a/src/blazingmq/_script_name.py
+++ b/src/blazingmq/_script_name.py
@@ -22,5 +22,6 @@
 def get_script_name() -> bytes:
     if getattr(sys.modules.get("__main__"), "__file__", ""):
         name = sys.modules["__main__"].__file__
+        assert name is not None  # help the typechecker
         return os.fsencode(name)
     return b"py:UNKNOWN"
diff --git a/src/blazingmq/_typing.py b/src/blazingmq/_typing.py
index 14bce6d..39cf633 100644
--- a/src/blazingmq/_typing.py
+++ b/src/blazingmq/_typing.py
@@ -13,13 +13,13 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from typing import Dict
+from typing import Mapping
 from typing import Union
 
 from ._enums import PropertyType
 
 PropertyValueType = Union[int, bytes, str]
 
-PropertyValueDict = Dict[str, PropertyValueType]
+PropertyValueDict = Mapping[str, PropertyValueType]
 
-PropertyTypeDict = Dict[str, PropertyType]
+PropertyTypeDict = Mapping[str, PropertyType]