diff --git a/Firmware/Marlin_main.cpp b/Firmware/Marlin_main.cpp
index 6de0e3df..68e4ad4d 100755
--- a/Firmware/Marlin_main.cpp
+++ b/Firmware/Marlin_main.cpp
@@ -9521,6 +9521,9 @@ Sigma_Exit:
 #endif //FILAMENT_SENSOR
 
 #endif //DEBUG_DCODES
+
+    default:
+        printf_P(PSTR("Unknown D code: %s\n"), cmdbuffer + bufindr + CMDHDRSIZE);
 	}
   }