diff --git a/src/object_cmd.cpp b/src/object_cmd.cpp
--- a/src/object_cmd.cpp
+++ b/src/object_cmd.cpp
@@ -741,7 +741,7 @@ static CommandCost TerraformTile_Object(
 			if (HasBit(spec->callback_mask, CBM_OBJ_AUTOSLOPE)) {
 				/* If the callback fails, allow autoslope. */
 				uint16 res = GetObjectCallback(CBID_OBJECT_AUTOSLOPE, 0, 0, spec, Object::GetByTile(tile), tile);
-				if ((res == 0) || (res == CALLBACK_FAILED)) return CommandCost(EXPENSES_CONSTRUCTION, _price[PR_BUILD_FOUNDATION]);
+				if (res == CALLBACK_FAILED || !ConvertBooleanCallback(spec->grf_prop.grffile, CBID_OBJECT_AUTOSLOPE, res)) return CommandCost(EXPENSES_CONSTRUCTION, _price[PR_BUILD_FOUNDATION]);
 			} else if (spec->enabled) {
 				/* allow autoslope */
 				return CommandCost(EXPENSES_CONSTRUCTION, _price[PR_BUILD_FOUNDATION]);