diff --git a/src/raspi_keyer.cpp b/src/raspi_keyer.cpp
index 89a5b85..f3acf9b 100644
--- a/src/raspi_keyer.cpp
+++ b/src/raspi_keyer.cpp
@@ -120,7 +120,7 @@ void cdc_task()
                 }
                 break;
             default:
-                printf("Unknown command.\n");
+                printf("Unknown command: %d.\n", buf[0]);
             }
         }
     }