diff --git a/cnpy.h b/cnpy.h
index 0d3bb4c..8171c8e 100644
--- a/cnpy.h
+++ b/cnpy.h
@@ -121,7 +121,7 @@ namespace cnpy {
         }
 
         std::vector<char> header = create_npy_header<T>(true_data_shape);
-        size_t nels = std::accumulate(shape.begin(),shape.end(),1,std::multiplies<size_t>());
+        size_t nels = std::accumulate(shape.begin(),shape.end(),static_cast<std::size_t>(1),std::multiplies<size_t>());
 
         fseek(fp,0,SEEK_SET);
         fwrite(&header[0],sizeof(char),header.size(),fp);
@@ -164,7 +164,7 @@ namespace cnpy {
 
         std::vector<char> npy_header = create_npy_header<T>(shape);
 
-        size_t nels = std::accumulate(shape.begin(),shape.end(),1,std::multiplies<size_t>());
+        size_t nels = std::accumulate(shape.begin(),shape.end(),static_cast<std::size_t>(1),std::multiplies<size_t>());
         size_t nbytes = nels*sizeof(T) + npy_header.size();
 
         //get the CRC of the data to be added