diff --git a/src/ltui/border.lua b/src/ltui/border.lua
index 6aec092..6c85d86 100644
--- a/src/ltui/border.lua
+++ b/src/ltui/border.lua
@@ -39,7 +39,7 @@ function border:init(name, bounds)
     view.init(self, name, bounds)
 
     -- check bounds
-    assert(self:width() > 2 and self:height() > 2, string.format("%s: too small!", self))
+    assert(self:width() > 2 and self:height() > 2, string.format("%s: too small!", tostring(self)))
 end
 
 -- draw border
diff --git a/src/ltui/window.lua b/src/ltui/window.lua
index a49cd11..4cb7da8 100644
--- a/src/ltui/window.lua
+++ b/src/ltui/window.lua
@@ -43,7 +43,7 @@ function window:init(name, bounds, title, shadow)
     panel.init(self, name, bounds)
 
     -- check bounds
-    assert(self:width() > 4 and self:height() > 3, string.format("%s: too small!", self))
+    assert(self:width() > 4 and self:height() > 3, string.format("%s: too small!", tostring(self)))
 
     -- insert shadow
     if shadow then