diff --git a/h5pyd/_hl/dataset.py b/h5pyd/_hl/dataset.py
index 97439bb..7f8038b 100644
--- a/h5pyd/_hl/dataset.py
+++ b/h5pyd/_hl/dataset.py
@@ -564,7 +564,7 @@ def size(self):
         """Numpy-style attribute giving the total dataset size"""
         if self._shape is None:
             return None
-        return numpy.prod(self._shape).item()
+        return numpy.prod(self._shape, dtype=numpy.int64).item()
 
     @property
     def nbytes(self):