diff --git a/Marlin/src/gcode/gcode.cpp b/Marlin/src/gcode/gcode.cpp
index 4b0a859943..0df31176c0 100644
--- a/Marlin/src/gcode/gcode.cpp
+++ b/Marlin/src/gcode/gcode.cpp
@@ -206,6 +206,8 @@ void GcodeSuite::process_parsed_command(
       #if ENABLED(INCH_MODE_SUPPORT)
         case 20: G20(); break;                                    // G20: Inch Mode
         case 21: G21(); break;                                    // G21: MM Mode
+      #else
+        case 21: NOOP; break;                                     // No error on unknown G21
       #endif
 
       #if ENABLED(G26_MESH_VALIDATION)