diff --git a/src/lua/core/fbgpu.lua b/src/lua/core/fbgpu.lua
index fcffddb..88c2ad9 100644
--- a/src/lua/core/fbgpu.lua
+++ b/src/lua/core/fbgpu.lua
@@ -147,7 +147,7 @@ function fbgpu.start()
     checkArg(3, w, "number")
     checkArg(4, h, "number")
     checkArg(5, ch, "string")
-    ch = ch:sub(1, 1)
+    ch = usub(ch, 1, 1)
     fb.fill(x-1, y-1, x+w-2, y+h-2, background, foreground, utf8.codepoint(ch))
     return true
   end
diff --git a/src/lua/core/winapigpu.lua b/src/lua/core/winapigpu.lua
index 506fec5..cc3b735 100644
--- a/src/lua/core/winapigpu.lua
+++ b/src/lua/core/winapigpu.lua
@@ -22,7 +22,10 @@ end
 local background = 0
 local foreground = 0
 
+local usub
+
 function wingpu.start()
+  usub = modules.sandbox.unicode.sub
   local gpu = {}
 
   function gpu.bind() return false, "This is static bound gpu" end
@@ -135,7 +138,7 @@ function wingpu.start()
     checkArg(3, w, "number")
     checkArg(4, h, "number")
     checkArg(5, ch, "string")
-    ch = ch:sub(1, 1)
+    ch = usub(ch, 1, 1)
     win.fill(x-1, y-1, x+w-2, y+h-2, background, foreground, utf8.codepoint(ch))
     return true
   end