diff --git a/src/data/simple_dmatrix.cc b/src/data/simple_dmatrix.cc
index 56185b03e84a..eee98a494e01 100644
--- a/src/data/simple_dmatrix.cc
+++ b/src/data/simple_dmatrix.cc
@@ -53,7 +53,7 @@ DMatrix* SimpleDMatrix::SliceCol(std::size_t start, std::size_t size) {
     auto& h_data = out_page.data.HostVector();
     auto& h_offset = out_page.offset.HostVector();
     size_t rptr{0};
-    for (auto i = 0; i < this->Info().num_row_; i++) {
+    for (bst_row_t i = 0; i < this->Info().num_row_; i++) {
       auto inst = batch[i];
       auto prev_size = h_data.size();
       std::copy_if(inst.begin(), inst.end(), std::back_inserter(h_data), [&](Entry e) {