diff --git a/tests/rigswr.c b/tests/rigswr.c
index e58e6a132..0caf9bb22 100644
--- a/tests/rigswr.c
+++ b/tests/rigswr.c
@@ -166,7 +166,7 @@ int main(int argc, char *argv[])
                 return 1;
             }
 
-            strncat(conf_parms, optarg, MAXCONFLEN - strlen(conf_parms));
+            strncat(conf_parms, optarg, MAXCONFLEN - strlen(conf_parms) - 1);
             break;
 
         case 'p':