diff --git a/tui_main.c b/tui_main.c
index a412b6e..d1263aa 100644
--- a/tui_main.c
+++ b/tui_main.c
@@ -386,7 +386,7 @@ int main(int argc, char** argv) {
   }
 
   if (margin_thickness < 0) {
-    fprintf(stderr, "Margins must be >= 0\n");
+    fprintf(stderr, "Margins must be >= 0.\n");
     usage();
     return 1;
   }