diff --git a/src/c_api/c_api.cc b/src/c_api/c_api.cc
index 0ba795a39..0870c6cc5 100644
--- a/src/c_api/c_api.cc
+++ b/src/c_api/c_api.cc
@@ -181,7 +181,7 @@ XGB_DLL int XGDMatrixSliceDMatrixEx(DMatrixHandle handle,
         << "slice does not support group structure";
   }
   DMatrix* dmat = static_cast<std::shared_ptr<DMatrix>*>(handle)->get();
-  *out = new std::shared_ptr<DMatrix>(dmat->Slice({idxset, len}));
+  *out = new std::shared_ptr<DMatrix>(dmat->Slice({idxset, static_cast<std::size_t>(len)}));
   API_END();
 }