diff --git a/Control/AthContainers/Root/PackedConverter.cxx b/Control/AthContainers/Root/PackedConverter.cxx
index 6abcaa9965260c5698f04570e8174fa83907921a..6dda03d11e354379ad45679a1905cbcedb54d1e3 100644
--- a/Control/AthContainers/Root/PackedConverter.cxx
+++ b/Control/AthContainers/Root/PackedConverter.cxx
@@ -24,7 +24,7 @@ namespace SG {
 PackedConverter::PackedConverter (const PackedParameters& parms)
   : m_parms (parms),
     m_packer (parms.nbits(),
-              parms.nmantissa(),
+              parms.nmantissa() + (parms.isSigned()?1:0),
               parms.scale(),
               parms.isSigned(),
               parms.rounding())