commit 687017bf2278042ed278159cc289f8c5648b4b02
Author: dprunier <dominique.prunier@watch4net.com>
Date:   Tue Jan 24 15:43:27 2012

    Fix for r465 to handle NULL strings properly (maps to "")

diff --git a/src/category.cpp b/src/category.cpp
index 17dcd74..3a4f9d4 100644
--- a/src/category.cpp
+++ b/src/category.cpp
@@ -141,7 +141,7 @@ ibis::category::selectStrings(const ibis::bitvector& mask) const {
 	    std::vector<std::string>* strings = new std::vector<std::string>();
 	    strings->reserve(keys->size());
 	    for (unsigned i = 0; i < keys->size(); ++i) {
-		strings->push_back(dic[(*keys)[i]]);
+		strings->push_back((*keys)[i] == 0 ? "" : dic[(*keys)[i]]);
 	    }
 	    return strings;
 	}