diff --git a/csrc/mask_api/src/mask.cpp b/csrc/mask_api/src/mask.cpp
index 495da2f..bb20b15 100644
--- a/csrc/mask_api/src/mask.cpp
+++ b/csrc/mask_api/src/mask.cpp
@@ -461,7 +461,7 @@ namespace mask_api
                 uint64_t m = R[0].m;
                 uint64_t i, a, b;
 
-                std::vector<uint> cnts(h*w+1);
+                std::vector<uint> cnts(h * w + 1);
                 for (a = 0; a < m; a++)
                 {
                     cnts[a] = R[0].cnts[a];
@@ -751,12 +751,14 @@ namespace mask_api
             }
         }
 
-        std::variant<std::vector<RLE>, std::vector<double>> _preproc(const py::object &pyobj)
+        std::tuple<std::variant<std::vector<RLE>, std::vector<double>>, size_t> _preproc(const py::object &pyobj)
         {
             std::string type = py::str(py::type::of(pyobj));
             if (type == "<class 'numpy.ndarray'>")
             {
-                return _preproc_bbox_array(pyobj);
+                std::vector<double> result = _preproc_bbox_array(pyobj);
+
+                return std::make_tuple(result, (size_t)(result.size() / 4));
             }
             else if (type == "<class 'list'>")
             {
@@ -764,7 +766,7 @@ namespace mask_api
 
                 if (pyobj_list.size() == 0)
                 {
-                    return std::vector<double>(0);
+                    return std::make_tuple(std::vector<double>(0), 0);
                 }
 
                 bool isbox = true;
@@ -799,11 +801,13 @@ namespace mask_api
 
                 if (isbox)
                 {
-                    return _preproc_bbox_array(pyobj);
+                    std::vector<double> result = _preproc_bbox_array(pyobj);
+                    return std::make_tuple(result, (size_t)(result.size() / 4));
                 }
                 else if (isrle)
                 {
-                    return _frString(pyobj.cast<std::vector<py::dict>>());
+                    std::vector<RLE> result = _frString(pyobj.cast<std::vector<py::dict>>());
+                    return std::make_tuple(result, (size_t)result.size());
                 }
                 else
                 {
@@ -820,23 +824,25 @@ namespace mask_api
         std::variant<py::array_t<double, py::array::f_style>, std::vector<double>> iou(const py::object &dt, const py::object &gt, const std::vector<int> &iscrowd)
         {
 
-            auto _dt = _preproc(dt);
-            auto _gt = _preproc(gt);
+            auto [_dt, m] = _preproc(dt);
+            auto [_gt, n] = _preproc(gt);
 
-            std::size_t crowd_length = iscrowd.size();
+            if (m == 0 || n == 0)
+            {
+                return std::vector<double>(0);
+            }
 
             if (_dt.index() != _gt.index())
             {
                 throw std::out_of_range("The dt and gt should have the same data type, either RLEs, list or np.ndarray");
             }
 
+            std::size_t crowd_length = iscrowd.size();
             std::vector<double> iou;
-            std::size_t m, n;
 
             if (std::holds_alternative<std::vector<double>>(_dt))
             {
                 std::vector<double> _gt_box = std::get<std::vector<double>>(_gt);
-                n = (std::size_t)(_gt_box.size() / 4);
                 if (crowd_length > 0 && crowd_length == n)
                 {
 
@@ -848,7 +854,6 @@ namespace mask_api
             else
             {
                 std::vector<RLE> _gt_rle = std::get<std::vector<RLE>>(_gt);
-                n = _gt_rle.size();
                 if (crowd_length > 0 && crowd_length == n)
                 {
                     std::vector<RLE> _dt_rle = std::get<std::vector<RLE>>(_dt);
@@ -862,10 +867,6 @@ namespace mask_api
                 printf("crowd_length=%zu, n=%zu\n", crowd_length, n);
                 throw std::out_of_range("iscrowd must have the same length as gt");
             }
-            if (m == 0 || n == 0)
-            {
-                return std::vector<double>(0);
-            }
             return py::array(iou.size(), iou.data()).reshape({m, n});
         }