diff --git a/tests/rigctlsync.c b/tests/rigctlsync.c
index c71d99851..a576b3af2 100644
--- a/tests/rigctlsync.c
+++ b/tests/rigctlsync.c
@@ -455,7 +455,7 @@ 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 'v':