diff --git a/Firmware/mmu.cpp b/Firmware/mmu.cpp
index b3465828..85beeb61 100755
--- a/Firmware/mmu.cpp
+++ b/Firmware/mmu.cpp
@@ -383,8 +383,9 @@ void mmu_loop(void)
 			//printf_P(PSTR("Eact: %d\n"), int(e_active()));
 			if (!mmu_finda && CHECK_FSENSOR && fsensor_enabled) {
 				fsensor_checkpoint_print();
-				ad_markDepleted(mmu_extruder);
-				if (lcd_autoDepleteEnabled() && !ad_allDepleted())
+				if (mmu_extruder != MMU_FILAMENT_UNKNOWN) // Can't deplete unknown extruder.
+                    ad_markDepleted(mmu_extruder);
+				if (lcd_autoDepleteEnabled() && !ad_allDepleted() && mmu_extruder != MMU_FILAMENT_UNKNOWN) // Can't auto if F=?
 				{
 				    enquecommand_front_P(PSTR("M600 AUTO")); //save print and run M600 command
 				}