diff --git a/src/hdf5_array.hpp b/src/hdf5_array.hpp
index 43c5221..f9835a1 100644
--- a/src/hdf5_array.hpp
+++ b/src/hdf5_array.hpp
@@ -202,6 +202,29 @@ namespace CosmoTool {
     }
   };
 
+  class hdf5_StringType
+  {
+  public:
+    H5::StrType type;
+    
+    hdf5_StringType()
+        : type(0, H5T_VARIABLE)
+    {
+    }
+    
+    static const hdf5_StringType *ctype()
+    {
+        static hdf5_StringType singleton;
+        return &singleton;
+    }
+  };
+
+  template<> struct get_hdf5_data_type<std::string> { 
+    static H5::DataType type() {
+      return hdf5_StringType::ctype()->type;
+    }
+  };
+
   class hdf5_BoolType
   {
   public:
@@ -326,7 +349,7 @@ namespace CosmoTool {
       H5::DataSpace dataspace = dataset.getSpace();
       std::vector<hsize_t> dimensions(data.num_dimensions());
       
-      if (dataspace.getSimpleExtentNdims() != data.num_dimensions())
+      if ((size_t)dataspace.getSimpleExtentNdims() != (size_t)data.num_dimensions())
         {
           throw InvalidDimensions();
         }