diff --git a/artiq/coredevice/comm_mgmt.py b/artiq/coredevice/comm_mgmt.py
index 9833d7520..870e2759d 100644
--- a/artiq/coredevice/comm_mgmt.py
+++ b/artiq/coredevice/comm_mgmt.py
@@ -160,7 +160,12 @@ class CommMgmt:
     def config_read(self, key):
         self._write_header(Request.ConfigRead)
         self._write_string(key)
-        self._read_expect(Reply.ConfigData)
+        ty = self._read_header()
+        if ty == Reply.Error:
+            raise IOError("Device failed to read config. The key may not exist.")
+        elif ty != Reply.ConfigData:
+            raise IOError("Incorrect reply from device: {} (expected {})".
+                          format(ty, Reply.ConfigData))
         return self._read_string()
 
     def config_write(self, key, value):