diff --git a/substrate/utils/prometheus/Cargo.toml b/substrate/utils/prometheus/Cargo.toml
index 7766904a57..16feedb2b5 100644
--- a/substrate/utils/prometheus/Cargo.toml
+++ b/substrate/utils/prometheus/Cargo.toml
@@ -22,3 +22,7 @@ derive_more = "0.99"
 async-std = { version = "1.6.5", features = ["unstable"] }
 tokio = "1.10"
 hyper = { version = "0.14.11", default-features = false, features = ["http1", "server", "tcp"] }
+
+[dev-dependencies]
+hyper = { version = "0.14.11", features = ["client"] }
+tokio = { version = "1.10", features = ["rt-multi-thread"] }
diff --git a/substrate/utils/prometheus/src/lib.rs b/substrate/utils/prometheus/src/lib.rs
index 9f6f6e8f22..5771b65567 100644
--- a/substrate/utils/prometheus/src/lib.rs
+++ b/substrate/utils/prometheus/src/lib.rs
@@ -137,12 +137,21 @@ mod known_os {
 		prometheus_addr: SocketAddr,
 		registry: Registry,
 	) -> Result<(), Error> {
-		use networking::Incoming;
 		let listener = async_std::net::TcpListener::bind(&prometheus_addr)
 			.await
 			.map_err(|_| Error::PortInUse(prometheus_addr))?;
 
-		log::info!("〽️ Prometheus exporter started at {}", prometheus_addr);
+		init_prometheus_with_listener(listener, registry).await
+	}
+
+	/// Init prometheus using the given listener.
+	pub(crate) async fn init_prometheus_with_listener(
+		listener: async_std::net::TcpListener,
+		registry: Registry,
+	) -> Result<(), Error> {
+		use networking::Incoming;
+
+		log::info!("〽️ Prometheus exporter started at {}", listener.local_addr()?);
 
 		let service = make_service_fn(move |_| {
 			let registry = registry.clone();
@@ -162,3 +171,46 @@ mod known_os {
 		result
 	}
 }
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+	use hyper::{Client, Uri};
+	use std::convert::TryFrom;
+
+	#[test]
+	fn prometheus_works() {
+		const METRIC_NAME: &str = "test_test_metric_name_test_test";
+
+		let runtime = tokio::runtime::Runtime::new().expect("Creates the runtime");
+
+		let listener = runtime
+			.block_on(async_std::net::TcpListener::bind("127.0.0.1:0"))
+			.expect("Creates listener");
+
+		let local_addr = listener.local_addr().expect("Returns the local addr");
+
+		let registry = Registry::default();
+		register(
+			prometheus::Counter::new(METRIC_NAME, "yeah").expect("Creates test counter"),
+			&registry,
+		)
+		.expect("Registers the test metric");
+
+		runtime.spawn(known_os::init_prometheus_with_listener(listener, registry));
+
+		runtime.block_on(async {
+			let client = Client::new();
+
+			let res = client
+				.get(Uri::try_from(&format!("http://{}/metrics", local_addr)).expect("Parses URI"))
+				.await
+				.expect("Requests metrics");
+
+			let buf = hyper::body::to_bytes(res).await.expect("Converts body to bytes");
+
+			let body = String::from_utf8(buf.to_vec()).expect("Converts body to String");
+			assert!(body.contains(&format!("{} 0", METRIC_NAME)));
+		});
+	}
+}
diff --git a/substrate/utils/prometheus/src/networking.rs b/substrate/utils/prometheus/src/networking.rs
index de1a1c41d6..a24216bd23 100644
--- a/substrate/utils/prometheus/src/networking.rs
+++ b/substrate/utils/prometheus/src/networking.rs
@@ -47,8 +47,8 @@ impl tokio::io::AsyncRead for TcpStream {
 		buf: &mut tokio::io::ReadBuf<'_>,
 	) -> Poll<Result<(), std::io::Error>> {
 		Pin::new(&mut Pin::into_inner(self).0)
-			.poll_read(cx, buf.initialized_mut())
-			.map_ok(drop)
+			.poll_read(cx, buf.initialize_unfilled())
+			.map_ok(|s| buf.set_filled(s))
 	}
 }