diff --git a/include/xgboost/c_api.h b/include/xgboost/c_api.h
index 2daa4203878b..ee32e2194654 100644
--- a/include/xgboost/c_api.h
+++ b/include/xgboost/c_api.h
@@ -168,7 +168,7 @@ XGB_DLL int XGDMatrixCreateFromCSREx(const size_t* indptr,
  * \return 0 when success, -1 when failure happens
  */
 XGB_DLL int XGDMatrixCreateFromCSR(char const *indptr, char const *indices, char const *data,
-                                   bst_ulong ncol, char const *config, DMatrixHandle *out);
+                                   size_t ncol, char const *config, DMatrixHandle *out);
 
 /*!
  * \brief Create a matrix from dense array.
@@ -1094,7 +1094,7 @@ XGB_DLL int XGBoosterPredictFromDense(BoosterHandle handle, char const *values,
  * \return 0 when success, -1 when failure happens
  */
 XGB_DLL int XGBoosterPredictFromCSR(BoosterHandle handle, char const *indptr, char const *indices,
-                                    char const *values, bst_ulong ncol, char const *config,
+                                    char const *values, size_t ncol, char const *config,
                                     DMatrixHandle m, bst_ulong const **out_shape,
                                     bst_ulong *out_dim, const float **out_result);
 
diff --git a/src/c_api/c_api.cc b/src/c_api/c_api.cc
index 87a30283f925..3ad5936b4cac 100644
--- a/src/c_api/c_api.cc
+++ b/src/c_api/c_api.cc
@@ -399,7 +399,7 @@ XGB_DLL int XGDMatrixCreateFromCSREx(const size_t* indptr,
 
 XGB_DLL int XGDMatrixCreateFromCSR(char const *indptr,
                                    char const *indices, char const *data,
-                                   xgboost::bst_ulong ncol,
+                                   size_t ncol,
                                    char const* c_json_config,
                                    DMatrixHandle* out) {
   API_BEGIN();
@@ -1022,7 +1022,7 @@ XGB_DLL int XGBoosterPredictFromDense(BoosterHandle handle, char const *array_in
 }
 
 XGB_DLL int XGBoosterPredictFromCSR(BoosterHandle handle, char const *indptr, char const *indices,
-                                    char const *data, xgboost::bst_ulong cols,
+                                    char const *data, size_t cols,
                                     char const *c_json_config, DMatrixHandle m,
                                     xgboost::bst_ulong const **out_shape,
                                     xgboost::bst_ulong *out_dim, const float **out_result) {
@@ -1037,7 +1037,7 @@ XGB_DLL int XGBoosterPredictFromCSR(BoosterHandle handle, char const *indptr, ch
   auto proxy = dynamic_cast<data::DMatrixProxy *>(p_m.get());
   CHECK(proxy) << "Invalid input type for inplace predict.";
   xgboost_CHECK_C_ARG_PTR(indptr);
-  proxy->SetCSRData(indptr, indices, data, cols, true);
+  proxy->SetCSRData(indptr, indices, data, static_cast<bst_feature_t>(cols), true);
   auto *learner = static_cast<xgboost::Learner *>(handle);
   InplacePredictImpl(p_m, c_json_config, learner, out_shape, out_dim, out_result);
   API_END();