diff --git a/modules/io/src/mol/star_writer.hh b/modules/io/src/mol/star_writer.hh
index 49728fcb77d93a2e183ba0667556ceca33ff1470..1a7d49f2d596e95c0915ad252c2de4ee6f95e7c6 100644
--- a/modules/io/src/mol/star_writer.hh
+++ b/modules/io/src/mol/star_writer.hh
@@ -115,7 +115,7 @@ public:
       }
     }
     if(string_value == "") {
-      value.value_ = ".";
+      value.value_ = "?";
     } else if(has_space) {
       value.value_ = "'" + string_value + "'";
     }