diff --git a/artiq/coredevice/comm_generic.py b/artiq/coredevice/comm_generic.py
index 8afa8c096..b44e1fee2 100644
--- a/artiq/coredevice/comm_generic.py
+++ b/artiq/coredevice/comm_generic.py
@@ -59,6 +59,9 @@ class _D2HMsgType(Enum):
 class UnsupportedDevice(Exception):
     pass
 
+class LoadError(Exception):
+    pass
+
 class RPCReturnValueError(ValueError):
     pass
 
@@ -265,7 +268,11 @@ class CommGeneric:
         self._write_header(_H2DMsgType.LOAD_KERNEL)
         self._write_bytes(kernel_library)
 
-        self._read_empty(_D2HMsgType.LOAD_COMPLETED)
+        self._read_header()
+        if self._read_type == _D2HMsgType.LOAD_FAILED:
+            raise LoadError(self._read_string())
+        else:
+            self._read_expect(_D2HMsgType.LOAD_COMPLETED)
 
     def run(self):
         self._write_empty(_H2DMsgType.RUN_KERNEL)
diff --git a/artiq/firmware/runtime/session.rs b/artiq/firmware/runtime/session.rs
index ba1c3544c..41c3b837a 100644
--- a/artiq/firmware/runtime/session.rs
+++ b/artiq/firmware/runtime/session.rs
@@ -2,6 +2,7 @@ use std::prelude::v1::*;
 use std::{mem, str};
 use std::cell::RefCell;
 use std::io::{self, Read, Write};
+use std::error::Error;
 use std::btree_set::BTreeSet;
 use {config, rtio_mgt, mailbox, rpc_queue, kernel};
 use logger_artiq::BufferLogger;
@@ -179,8 +180,10 @@ unsafe fn kern_load(io: &Io, session: &mut Session, library: &[u8]) -> io::Resul
                 session.kernel_state = KernelState::Loaded;
                 Ok(())
             }
-            &kern::LoadReply(Err(error)) =>
-                unexpected!("cannot load kernel: {}", error),
+            &kern::LoadReply(Err(error)) => {
+                Err(io::Error::new(io::ErrorKind::Other,
+                                   format!("cannot load kernel: {}", error)))
+            }
             other =>
                 unexpected!("unexpected reply from kernel CPU: {:?}", other)
         }
@@ -259,9 +262,9 @@ fn process_host_message(io: &Io,
         host::Request::LoadKernel(kernel) =>
             match unsafe { kern_load(io, session, &kernel) } {
                 Ok(()) => host_write(stream, host::Reply::LoadCompleted),
-                Err(_) => {
-                    try!(kern_acknowledge());
-                    host_write(stream, host::Reply::LoadFailed)
+                Err(error) => {
+                    host_write(stream, host::Reply::LoadFailed(error.description()));
+                    kern_acknowledge()
                 }
             },
 
diff --git a/artiq/firmware/runtime/session_proto.rs b/artiq/firmware/runtime/session_proto.rs
index e83b955f7..3f9df8864 100644
--- a/artiq/firmware/runtime/session_proto.rs
+++ b/artiq/firmware/runtime/session_proto.rs
@@ -92,7 +92,7 @@ pub enum Reply<'a> {
     ClockSwitchFailed,
 
     LoadCompleted,
-    LoadFailed,
+    LoadFailed(&'a str),
 
     KernelFinished,
     KernelStartupFailed,
@@ -141,8 +141,9 @@ impl<'a> Reply<'a> {
             Reply::LoadCompleted => {
                 try!(write_u8(writer, 5));
             },
-            Reply::LoadFailed => {
+            Reply::LoadFailed(reason) => {
                 try!(write_u8(writer, 6));
+                try!(write_string(writer, reason));
             },
 
             Reply::KernelFinished => {