diff --git a/sse_starlette/sse.py b/sse_starlette/sse.py
index f52b93a..ceb4460 100644
--- a/sse_starlette/sse.py
+++ b/sse_starlette/sse.py
@@ -257,7 +257,7 @@ async def _ping(self, send: Send) -> None:
             if self.ping_message_factory:
                 assert isinstance(self.ping_message_factory, Callable)  # type: ignore  # https://github.com/python/mypy/issues/6864
             ping = (
-                ServerSentEvent(datetime.utcnow(), event="ping").encode()
+                ServerSentEvent(datetime.utcnow(), event=": ping").encode()
                 if self.ping_message_factory is None
                 else ensure_bytes(self.ping_message_factory())
             )