diff --git a/src/libhictk/balancing/include/hictk/balancing/impl/sparse_matrix_impl.hpp b/src/libhictk/balancing/include/hictk/balancing/impl/sparse_matrix_impl.hpp
index 455ec685..0c917dab 100644
--- a/src/libhictk/balancing/include/hictk/balancing/impl/sparse_matrix_impl.hpp
+++ b/src/libhictk/balancing/include/hictk/balancing/impl/sparse_matrix_impl.hpp
@@ -136,8 +136,8 @@ inline void SparseMatrix::shrink_to_fit() noexcept {
 
 inline void SparseMatrix::finalize() { shrink_to_fit(); }
 
-inline const std::vector<std::size_t>& SparseMatrix::bin1_ids() const noexcept { return _bin1_ids; }
-inline const std::vector<std::size_t>& SparseMatrix::bin2_ids() const noexcept { return _bin2_ids; }
+inline const std::vector<std::uint64_t>& SparseMatrix::bin1_ids() const noexcept { return _bin1_ids; }
+inline const std::vector<std::uint64_t>& SparseMatrix::bin2_ids() const noexcept { return _bin2_ids; }
 inline const std::vector<double>& SparseMatrix::counts() const noexcept { return _counts; }
 
 inline void SparseMatrix::push_back(std::uint64_t bin1_id, std::uint64_t bin2_id, double count,
diff --git a/src/libhictk/balancing/include/hictk/balancing/sparse_matrix.hpp b/src/libhictk/balancing/include/hictk/balancing/sparse_matrix.hpp
index af8855bb..f9e6b669 100644
--- a/src/libhictk/balancing/include/hictk/balancing/sparse_matrix.hpp
+++ b/src/libhictk/balancing/include/hictk/balancing/sparse_matrix.hpp
@@ -80,8 +80,8 @@ class SparseMatrix {
   void shrink_to_fit() noexcept;
   void finalize();
 
-  [[nodiscard]] const std::vector<std::size_t>& bin1_ids() const noexcept;
-  [[nodiscard]] const std::vector<std::size_t>& bin2_ids() const noexcept;
+  [[nodiscard]] const std::vector<std::uint64_t>& bin1_ids() const noexcept;
+  [[nodiscard]] const std::vector<std::uint64_t>& bin2_ids() const noexcept;
   [[nodiscard]] const std::vector<double>& counts() const noexcept;
 
   void push_back(std::uint64_t bin1_id, std::uint64_t bin2_id, double count,