diff --git a/src/serena/connection.py b/src/serena/connection.py
index 8ca3544..1e44cbb 100644
--- a/src/serena/connection.py
+++ b/src/serena/connection.py
@@ -86,7 +86,7 @@ class HeartbeatStatistics:
     prev_heartbeat_mn: int = attr.ib(default=-1)
 
     #: The current heartbeat time, in monotonic nanoseconds.
-    cur_heartbeat_mn: int = attr.ib(default=None)
+    cur_heartbeat_mn: int = attr.ib(default=-1)
 
     #: The previous heartbeat time, in wall clock time.
     prev_heartbeat_time: datetime = attr.ib(default=None)
@@ -100,7 +100,7 @@ def interval(self) -> int | None:
         Returns the interval between two heartbeats in nanoseconds.
         """
 
-        if self.prev_heartbeat_mn == -1:
+        if self.prev_heartbeat_mn == -1 or self.cur_heartbeat_mn == -1:
             return None
 
         return self.cur_heartbeat_mn - self.prev_heartbeat_mn
diff --git a/tests/__init__.py b/tests/__init__.py
index dd69cec..36547ce 100644
--- a/tests/__init__.py
+++ b/tests/__init__.py
@@ -17,7 +17,10 @@
 @asynccontextmanager
 async def _open_connection(**kwargs: Any) -> AsyncGenerator[AMQPConnection, None]:  # type: ignore
     async with serena.open_connection(
-        AMQP_HOST, port=AMQP_PORT, username=AMQP_USERNAME, password=AMQP_PASSWORD,
-        **kwargs  # type: ignore
+        AMQP_HOST,
+        port=AMQP_PORT,
+        username=AMQP_USERNAME,
+        password=AMQP_PASSWORD,
+        **kwargs,  # type: ignore
     ) as conn:
         yield conn
diff --git a/tests/channel/test_exchange.py b/tests/channel/test_exchange.py
index bf18c32..66eb689 100644
--- a/tests/channel/test_exchange.py
+++ b/tests/channel/test_exchange.py
@@ -27,12 +27,12 @@ async def test_ex_declaration_invalid_type():
     Tests declaration with an invalid type.
     """
 
-    async with _open_connection() as conn:
-        with pytest.raises(UnexpectedCloseError) as e:
-            async with conn.open_channel() as channel:
-                await channel.exchange_declare(name="invalid", type="invalid")
+    with pytest.raises(UnexpectedCloseError) as e:
+        async with _open_connection() as conn:
+                async with conn.open_channel() as channel:
+                    await channel.exchange_declare(name="invalid", type="invalid")
 
-        assert e.value.reply_code in (ReplyCode.command_invalid, ReplyCode.precondition_failed)
+    assert e.value.reply_code in (ReplyCode.command_invalid, ReplyCode.precondition_failed)
 
 
 async def test_ex_delete():
diff --git a/tests/channel/test_queue.py b/tests/channel/test_queue.py
index def82e4..a254205 100644
--- a/tests/channel/test_queue.py
+++ b/tests/channel/test_queue.py
@@ -90,13 +90,13 @@ async def consumer(channel: Channel, queue_name: str, *, task_status: TaskStatus
             await anyio.sleep_forever()
 
     async with _open_connection() as conn:
-        with pytest.raises(ExceptionGroup[UnexpectedCloseError]) as e:
+        with pytest.raises(ExceptionGroup) as e:  # type: ignore
             async with conn.open_channel() as channel, anyio.create_task_group() as group:
                 queue = await channel.queue_declare(exclusive=True)
                 await group.start(consumer, channel, queue.name)
                 await channel.queue_delete(queue.name, if_unused=True)
 
-        assert isinstance(unwrapped := e.value.exceptions[0], UnexpectedCloseError)
+        assert isinstance(unwrapped := e.value.exceptions[0], UnexpectedCloseError)  # type: ignore
         assert unwrapped.reply_code == ReplyCode.precondition_failed
 
 
diff --git a/tests/test_connection.py b/tests/test_connection.py
index 8914376..2a8ce0c 100644
--- a/tests/test_connection.py
+++ b/tests/test_connection.py
@@ -4,7 +4,7 @@
 from serena.enums import ReplyCode
 from serena.exc import UnexpectedCloseError
 
-from tests import _open_connection
+from tests import _open_connection, AMQP_HOST, AMQP_PORT, AMQP_USERNAME
 
 pytestmark = pytest.mark.anyio
 
@@ -50,7 +50,9 @@ async def test_bad_authentication():
     """
 
     with pytest.raises(UnexpectedCloseError) as e:
-        async with _open_connection(password="not the right password!"):
+        async with open_connection(
+            AMQP_HOST, port=AMQP_PORT, username=AMQP_USERNAME, password="not the right password!"
+        ):
             pass
 
     assert e.value.reply_code == ReplyCode.access_refused