diff --git a/reading_machine/src/Config.cpp b/reading_machine/src/Config.cpp
index 3b6041bed5c186adefb30c43a9b495509d607f73..faeb04fc4277d9e659c204c7135ce9c12f0eedb9 100644
--- a/reading_machine/src/Config.cpp
+++ b/reading_machine/src/Config.cpp
@@ -136,7 +136,12 @@ void Config::printForDebug(FILE * dest) const
   for (auto & s : stack)
   {
     if (hasColIndex(idColName))
-      stackStr += getLastNotEmptyConst(idColName, s);
+    {
+      if (has(idColName, s, 0))
+        stackStr += getLastNotEmptyConst(idColName, s);
+      else
+        stackStr += "?";
+    }
     else
       stackStr += std::to_string(s);
     stackStr += ",";