diff --git a/src/raspberrypi/rascsi.cpp b/src/raspberrypi/rascsi.cpp
index 675cbef9..c2994666 100644
--- a/src/raspberrypi/rascsi.cpp
+++ b/src/raspberrypi/rascsi.cpp
@@ -1589,6 +1589,10 @@ static void *MonThread(void *param)
 				}
 
 				default: {
+					LOGTRACE("Received unknown command %d", command.operation());
+
+					ReturnStatus(fd, false, "Unknown command", UNKNOWN_OPERATION);
+
 					// Wait until we become idle
 					while (active) {
 						usleep(500 * 1000);
diff --git a/src/raspberrypi/rascsi_interface.proto b/src/raspberrypi/rascsi_interface.proto
index 1039e059..92482fb5 100644
--- a/src/raspberrypi/rascsi_interface.proto
+++ b/src/raspberrypi/rascsi_interface.proto
@@ -201,8 +201,10 @@ message PbOperationInfo {
 enum PbErrorCode {
     // No error code available
     NO_ERROR_CODE = 0;
+    // The client sent an operation code not supported by this server
+    UNKNOWN_OPERATION = 1;
     // Authentication/Authorization error
-    UNAUTHORIZED = 1;
+    UNAUTHORIZED = 2;
 }
 
 // The supported file extensions mapped to their respective device types