diff --git a/lang/english.txt b/lang/english.txt
index 467fc8523b..793d83e13e 100644
--- a/lang/english.txt
+++ b/lang/english.txt
@@ -2906,6 +2906,8 @@ STR_REPLACE_REMOVE_WAGON_HELP                                   :{BLACK}Make aut
 STR_REPLACE_ENGINE_WAGON_SELECT                                 :{BLACK}Replacing: {ORANGE}{SKIP}{SKIP}{STRING}
 STR_REPLACE_ENGINE_WAGON_SELECT_HELP                            :{BLACK} EXPERIMENTAL FEATURE {}Switch between engine and wagon replacement windows.{}Wagon replacement will only be done if the new wagon can be refitted into carrying the same type of cargo as the old one. This is checked for each wagon when the actual replacement takes place.
 STR_ENGINE_NOT_BUILDABLE                                        :{WHITE}Engine is not buildable
+STR_AUTOREPLACE_FAILED                                          :{WHITE}BUG: Autoreplace refit failure
+STR_AUTOREPLACE_REFIT_FAILURE                                   :{WHITE}{STRING} replaced into {STRING} failed to refit to {STRING}
 
 STR_ENGINES                                                     :Engines
 STR_WAGONS                                                      :Wagons
diff --git a/vehicle.c b/vehicle.c
index 6d16bf60a1..17cc63f1b3 100644
--- a/vehicle.c
+++ b/vehicle.c
@@ -1778,9 +1778,18 @@ static int32 ReplaceVehicle(Vehicle **w, byte flags, int32 total_cost)
 
 		/* refit if needed */
 		if (replacement_cargo_type != CT_NO_REFIT) {
-			int32 temp_cost = DoCommand(0, new_v->index, replacement_cargo_type, DC_EXEC, CMD_REFIT_VEH(new_v->type));
-			assert(!CmdFailed(temp_cost)); // assert failure here indicates a bug in GetNewCargoTypeForReplace()
+			if (CmdFailed(DoCommand(0, new_v->index, replacement_cargo_type, DC_EXEC, CMD_REFIT_VEH(new_v->type)))) {
+				/* We should not be here
+				 * being here shows a failure indicates a bug in GetNewCargoTypeForReplace() or incorrect estimation costs */
+				SetDParam(0, GetCustomEngineName(old_v->engine_type));
+				SetDParam(1, GetCustomEngineName(new_v->engine_type));
+				SetDParam(2, _cargoc.names_s[replacement_cargo_type]);
+				ShowInfo("Error: Autoreplace failed to refit (bug)");
+				ShowErrorMessage(STR_AUTOREPLACE_REFIT_FAILURE, STR_AUTOREPLACE_FAILED, 0 ,0);
+				NOT_REACHED(); // stop debug builds at this error for easier debugging
+			}
 		}
+
 		if (new_v->type == VEH_Train && HASBIT(old_v->u.rail.flags, VRF_REVERSE_DIRECTION) && !IsMultiheaded(new_v) && !(new_v->next != NULL && IsArticulatedPart(new_v->next))) {
 			// we are autorenewing to a single engine, so we will turn it as the old one was turned as well
 			SETBIT(new_v->u.rail.flags, VRF_REVERSE_DIRECTION);