diff --git a/reading_machine/src/Config.cpp b/reading_machine/src/Config.cpp
index fe65a0698d9627a758a9d754f7b1b0d303d05a77..0e5f6d0ea5579fd3b8b07b8711a0be9a1d03691e 100644
--- a/reading_machine/src/Config.cpp
+++ b/reading_machine/src/Config.cpp
@@ -71,8 +71,12 @@ void Config::print(FILE * dest) const
     }
     for (unsigned int i = 0; i < getNbColumns()-1; i++)
     {
-      auto & colContent = isPredicted(getColName(i)) ? getLastNotEmptyHypConst(i, getFirstLineIndex()+line) : getLastNotEmptyConst(i, line);
-      fmt::print(dest, "{}{}", colContent, i < getNbColumns()-2 ? "\t" : "\n");
+      auto & colContent = isPredicted(getColName(i)) ? getLastNotEmptyHypConst(i, getFirstLineIndex()+line) : getLastNotEmptyConst(i, getFirstLineIndex()+line);
+      std::string valueToPrint = colContent;
+      if (valueToPrint.empty())
+        valueToPrint = "_";
+
+      fmt::print(dest, "{}{}", valueToPrint, i < getNbColumns()-2 ? "\t" : "\n");
     }
     if (getLastNotEmptyConst(EOSColName, getFirstLineIndex()+line) == EOSSymbol1)
       fmt::print(dest, "\n");