diff --git a/examples/block-download/src/main.rs b/examples/block-download/src/main.rs
index 6fb45e8..9dec596 100644
--- a/examples/block-download/src/main.rs
+++ b/examples/block-download/src/main.rs
@@ -16,7 +16,7 @@ use std::net::TcpStream;
 struct BlockPrinter;
 
 impl Observer for BlockPrinter {
-    fn on_block_received(&self, body: Vec<u8>) -> Result<(), Box<dyn std::error::Error>> {
+    fn on_block_received(&mut self, body: Vec<u8>) -> Result<(), Box<dyn std::error::Error>> {
         println!("{}", hex::encode(&body));
         println!("----------");
         Ok(())
diff --git a/pallas-miniprotocols/src/blockfetch/mod.rs b/pallas-miniprotocols/src/blockfetch/mod.rs
index 6043010..3fd1d60 100644
--- a/pallas-miniprotocols/src/blockfetch/mod.rs
+++ b/pallas-miniprotocols/src/blockfetch/mod.rs
@@ -88,7 +88,7 @@ impl<'b> Decode<'b> for Message {
 }
 
 pub trait Observer {
-    fn on_block_received(&self, body: Vec<u8>) -> Result<(), Box<dyn std::error::Error>> {
+    fn on_block_received(&mut self, body: Vec<u8>) -> Result<(), Box<dyn std::error::Error>> {
         log::debug!("block received, sice: {}", body.len());
         Ok(())
     }
@@ -148,7 +148,7 @@ where
         })
     }
 
-    fn on_block(self, body: Vec<u8>) -> Transition<Self> {
+    fn on_block(mut self, body: Vec<u8>) -> Transition<Self> {
         log::debug!("received block body, size {}", body.len());
 
         self.observer.on_block_received(body)?;
@@ -287,7 +287,7 @@ where
         })
     }
 
-    fn on_block(self, body: Vec<u8>) -> Transition<Self> {
+    fn on_block(mut self, body: Vec<u8>) -> Transition<Self> {
         log::debug!("received block body, size {}", body.len());
 
         self.observer.on_block_received(body)?;