diff --git a/include/prefix.cpp b/include/prefix.cpp
index 5f02ce66b..c3c89baa3 100755
--- a/include/prefix.cpp
+++ b/include/prefix.cpp
@@ -318,7 +318,9 @@ void TPrefix::put()
 
 bool TPrefix::test(long codditta) const
 {
-  TString16 s; s.format("%05lda", codditta);
+  TString16 s("com");
+  if (codditta > 0L)
+    s.format("%05lda", codditta);
   return test(s);
 }
 
@@ -335,7 +337,9 @@ bool TPrefix::set_codditta(long codditta, bool force)
 {
   if (force || test(codditta))
   {
-    TString16 s; s.format("%05lda", codditta); 
+    TString16 s("com");
+    if (codditta > 0L) 
+      s.format("%05lda", codditta); 
     set(s, force);
     put();
     return TRUE;