diff --git a/include/maskfld.cpp b/include/maskfld.cpp
index 86a530e2d..380fe2cb5 100755
--- a/include/maskfld.cpp
+++ b/include/maskfld.cpp
@@ -1,4 +1,4 @@
-//      $Id: maskfld.cpp,v 1.82 1995-02-28 10:30:44 pirro Exp $
+//      $Id: maskfld.cpp,v 1.83 1995-03-13 11:48:49 pirro Exp $
 #include <xvt.h>
 
 #include <applicat.h>
@@ -2598,7 +2598,7 @@ bool TReal_field::on_key(KEY key)
       if (_flags.firm)
       {                         
         const long f = atol(n);
-        if (prefhndl->exist(f))
+        if (f > 0 && prefhndl->exist(f))
           main_app().set_firm(f);
       }  
     }