diff --git a/src/client.rs b/src/client.rs
index 45b7c16..93b7efb 100644
--- a/src/client.rs
+++ b/src/client.rs
@@ -273,16 +273,16 @@ impl Subscriber {
         }
 
         // Read the response
-        for channel in &channels {
+        for _channel in &channels {
             let response = self.read_response().await?;
             match response {
                 Frame::Array(ref frame) => match frame.as_slice() {
-                    [unsubscribe, uchannel]
-                        if &unsubscribe.to_string() == "unsubscribe"
-                            && &uchannel.to_string() == channel =>
-                    {
-                        self.subscribed_channels.remove(&uchannel.to_string());
-                    }
+                    [unsubscribe, uchannel] if &unsubscribe.to_string() == "unsubscribe" => {
+                        //unsubscribed channel should exist in the subscribed list at this point
+                        if self.subscribed_channels.remove(&uchannel.to_string()) == false {
+                            return Err(response.to_error());
+                        }
+                    },
                     _ => return Err(response.to_error()),
                 },
                 frame => return Err(frame.to_error()),
diff --git a/src/cmd/mod.rs b/src/cmd/mod.rs
index e83c15f..a437a78 100644
--- a/src/cmd/mod.rs
+++ b/src/cmd/mod.rs
@@ -10,6 +10,9 @@ pub use set::Set;
 mod subscribe;
 pub use subscribe::{Subscribe, Unsubscribe};
 
+mod unknown;
+pub use unknown::Unknown;
+
 use crate::{Connection, Db, Frame, Parse, ParseError, Shutdown};
 
 #[derive(Debug)]
@@ -19,6 +22,7 @@ pub(crate) enum Command {
     Set(Set),
     Subscribe(Subscribe),
     Unsubscribe(Unsubscribe),
+    Unknown(Unknown)
 }
 
 impl Command {
@@ -33,7 +37,10 @@ impl Command {
             "set" => Command::Set(Set::parse_frames(&mut parse)?),
             "subscribe" => Command::Subscribe(Subscribe::parse_frames(&mut parse)?),
             "unsubscribe" => Command::Unsubscribe(Unsubscribe::parse_frames(&mut parse)?),
-            _ => return Err(ParseError::UnknownCommand(command_name)),
+            _ => {
+                parse.next_string()?;
+                Command::Unknown(Unknown::new(command_name))
+            },
         };
 
         parse.finish()?;
@@ -53,9 +60,21 @@ impl Command {
             Publish(cmd) => cmd.apply(db, dst).await,
             Set(cmd) => cmd.apply(db, dst).await,
             Subscribe(cmd) => cmd.apply(db, dst, shutdown).await,
+            Unknown(cmd) => cmd.apply(dst).await,
             // `Unsubscribe` cannot be applied. It may only be received from the
             // context of a `Subscribe` command.
             Unsubscribe(_) => unimplemented!(),
         }
     }
+
+    pub(crate) fn get_name(&self) -> &str {
+        match self {
+            Command::Get(_) => "get",
+            Command::Publish(_) => "pub",
+            Command::Set(_) => "set",
+            Command::Subscribe(_) => "subscribe",
+            Command::Unsubscribe(_) => "unsubscribe",
+            Command::Unknown(cmd) => &cmd.command_name,
+        }
+    }
 }
diff --git a/src/cmd/subscribe.rs b/src/cmd/subscribe.rs
index 7e6cdef..a2c924b 100644
--- a/src/cmd/subscribe.rs
+++ b/src/cmd/subscribe.rs
@@ -1,4 +1,4 @@
-use crate::cmd::{Parse, ParseError};
+use crate::cmd::{Parse, ParseError, Unknown};
 use crate::{Command, Connection, Db, Frame, Shutdown};
 
 use bytes::Bytes;
@@ -134,9 +134,9 @@ impl Subscribe {
                                 dst.write_frame(&response).await?;
                             }
                         }
-                        _ => {
-                            // TODO: received invalid command
-                            unimplemented!();
+                        command => {
+                            let cmd = Unknown::new(command.get_name());
+                            cmd.apply(dst).await?;
                         }
                     }
                 }
diff --git a/src/cmd/unknown.rs b/src/cmd/unknown.rs
new file mode 100644
index 0000000..20437b2
--- /dev/null
+++ b/src/cmd/unknown.rs
@@ -0,0 +1,26 @@
+use crate::{Connection, Frame};
+
+use tracing::{debug, instrument};
+
+#[derive(Debug)]
+pub struct Unknown {
+    pub command_name: String,
+}
+
+impl Unknown {
+    /// Create a new `Unknown` command which responds to unknown commands
+    /// issued by clients
+    pub(crate) fn new(key: impl ToString) -> Unknown {
+        Unknown { command_name: key.to_string() }
+    }
+
+    #[instrument(skip(self, dst))]
+    pub(crate) async fn apply(self, dst: &mut Connection) -> crate::Result<()> {
+        let response = Frame::Error(format!("ERR unknown command '{}'", self.command_name));
+
+        debug!(?response);
+
+        dst.write_frame(&response).await?;
+        Ok(())
+    }
+}
diff --git a/src/parse.rs b/src/parse.rs
index d6c8125..e77ab86 100644
--- a/src/parse.rs
+++ b/src/parse.rs
@@ -13,7 +13,6 @@ pub(crate) struct Parse {
 pub(crate) enum ParseError {
     EndOfStream,
     Invalid,
-    UnknownCommand(String),
 }
 
 impl Parse {
@@ -85,7 +84,6 @@ impl fmt::Display for ParseError {
         let msg = match self {
             ParseError::EndOfStream => "end of stream".to_string(),
             ParseError::Invalid => "invalid".to_string(),
-            ParseError::UnknownCommand(cmd) => format!("unknown command `{}`", cmd),
         };
         write!(f, "{}", &msg)
     }
diff --git a/tests/server.rs b/tests/server.rs
index bb94f31..f5ffa04 100644
--- a/tests/server.rs
+++ b/tests/server.rs
@@ -98,6 +98,86 @@ async fn key_value_timeout() {
     assert_eq!(b"$-1\r\n", &response);
 }
 
+// In this case we test that server responds acurately to
+// SUBSCRIBE and UNSUBSCRIBE commands
+#[tokio::test]
+async fn subscribe_unsubscribe() {
+    let (addr, _handle) = start_server().await;
+
+    let mut stream = TcpStream::connect(addr).await.unwrap();
+
+    // send SUBSCRIBE command
+    stream.write_all(b"*2\r\n$9\r\nsubscribe\r\n$5\r\nhello\r\n").await.unwrap();
+
+    // Read response
+    let mut response = [0; 30];
+
+    stream.read_exact(&mut response).await.unwrap();
+
+    assert_eq!(b"*2\r\n$9\r\nsubscribe\r\n$5\r\nhello\r\n", &response);
+
+    // send UNSUBSCRIBE command
+    stream.write_all(b"*2\r\n$11\r\nunsubscribe\r\n$5\r\nhello\r\n").await.unwrap();
+
+    let mut response = [0; 33];
+
+    stream.read_exact(&mut response).await.unwrap();
+
+    assert_eq!(b"*2\r\n$11\r\nunsubscribe\r\n", &response[0..22]);
+    assert_eq!(b"$5\r\nhello\r\n", &response[22..33]);
+}
+
+// In this case we test that server Responds with an Error message if a client
+// sends an unknown command
+#[tokio::test]
+async fn send_error_unknown_command() {
+    let (addr, _handle) = start_server().await;
+
+    // Establish a connection to the server
+    let mut stream = TcpStream::connect(addr).await.unwrap();
+
+    // Get a key, data is missing
+    stream.write_all(b"*2\r\n$3\r\nFOO\r\n$5\r\nhello\r\n").await.unwrap();
+
+    let mut response = [0; 28];
+
+    stream.read_exact(&mut response).await.unwrap();
+
+    assert_eq!(b"-ERR unknown command \'foo\'\r\n", &response);
+}
+
+// In this case we test that server Responds with an Error message if a client
+// sends an GET or SET command after a SUBSCRIBE
+#[tokio::test]
+async fn send_error_get_set_after_subscribe() {
+    let (addr, _handle) = start_server().await;
+
+    let mut stream = TcpStream::connect(addr).await.unwrap();
+
+    // send SUBSCRIBE command
+    stream.write_all(b"*2\r\n$9\r\nsubscribe\r\n$5\r\nhello\r\n").await.unwrap();
+
+    let mut response = [0; 30];
+
+    stream.read_exact(&mut response).await.unwrap();
+
+    assert_eq!(b"*2\r\n$9\r\nsubscribe\r\n$5\r\nhello\r\n", &response);
+
+    stream.write_all(b"*3\r\n$3\r\nSET\r\n$5\r\nhello\r\n$5\r\nworld\r\n").await.unwrap();
+
+    let mut response = [0; 28];
+
+    stream.read_exact(&mut response).await.unwrap();
+    assert_eq!(b"-ERR unknown command \'set\'\r\n", &response);
+
+    stream.write_all(b"*2\r\n$3\r\nGET\r\n$5\r\nhello\r\n").await.unwrap();
+
+    let mut response = [0; 28];
+
+    stream.read_exact(&mut response).await.unwrap();
+    assert_eq!(b"-ERR unknown command \'get\'\r\n", &response);
+}
+
 async fn start_server() -> (SocketAddr, JoinHandle<mini_redis::Result<()>>) {
     let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
     let addr = listener.local_addr().unwrap();