diff --git a/src/proto/framed_read.rs b/src/proto/framed_read.rs
index e4445a5..3fbc509 100644
--- a/src/proto/framed_read.rs
+++ b/src/proto/framed_read.rs
@@ -136,7 +136,8 @@ impl<T> FramedRead<T> {
                 }
             }
             Kind::Unknown => {
-                unimplemented!()
+                // Unknown frames are ignored
+                return Ok(None);
             }
         };
 
diff --git a/src/server.rs b/src/server.rs
index 1fee874..422c4f4 100644
--- a/src/server.rs
+++ b/src/server.rs
@@ -281,8 +281,8 @@ impl<T: AsyncRead> Future for ReadPreface<T> {
             let n = try_nb!(self.inner.as_mut().unwrap().read(&mut buf[..rem]));
 
             if PREFACE[self.pos..self.pos+n] != buf[..n] {
-                // TODO: Invalid connection preface
-                unimplemented!();
+                // TODO: Should this just write the GO_AWAY frame directly?
+                return Err(ProtocolError.into());
             }
 
             self.pos += n;