diff --git a/src/hdf5_array.hpp b/src/hdf5_array.hpp
index c15c499..43c5221 100644
--- a/src/hdf5_array.hpp
+++ b/src/hdf5_array.hpp
@@ -286,7 +286,7 @@ namespace CosmoTool {
 
   template<typename ArrayType>
   void hdf5_check_array(ArrayType& data, std::vector<hsize_t>& dims) {
-    for (long i = 0; i < data.num_dimensions(); i++) {
+    for (size_t i = 0; i < data.num_dimensions(); i++) {
         if (data.shape()[i] != dims[i]) {
           throw InvalidDimensions();
         }
@@ -295,7 +295,7 @@ namespace CosmoTool {
 
   template<typename ArrayType>
   void hdf5_weak_check_array(ArrayType& data, std::vector<hsize_t>& dims) {
-    for (long i = 0; i < data.num_dimensions(); i++) {
+    for (size_t i = 0; i < data.num_dimensions(); i++) {
         if (data.index_bases()[i] < 0) {
           // Negative indexes are not supported right now.
           throw InvalidDimensions();