diff --git a/bin/mbus-serial-scan-secondary.c b/bin/mbus-serial-scan-secondary.c
index bb04e2f..b382f4a 100755
--- a/bin/mbus-serial-scan-secondary.c
+++ b/bin/mbus-serial-scan-secondary.c
@@ -128,7 +128,7 @@ main(int argc, char **argv)
         return 1;
     }
  
-    if (mbus_is_secondary_address(addr_mask))
+    if (mbus_is_secondary_address(addr_mask) == 0)
     {
         fprintf(stderr, "Misformatted secondary address mask. Must be 16 character HEX number.\n");
         free(addr_mask);