diff --git a/BizHawk.Emulation/Consoles/Nintendo/NES/Boards/GxROM.cs b/BizHawk.Emulation/Consoles/Nintendo/NES/Boards/GxROM.cs
index 1a3c1bb470..d3a4edbce7 100644
--- a/BizHawk.Emulation/Consoles/Nintendo/NES/Boards/GxROM.cs
+++ b/BizHawk.Emulation/Consoles/Nintendo/NES/Boards/GxROM.cs
@@ -41,7 +41,7 @@ namespace BizHawk.Emulation.Consoles.Nintendo
 					return false;
 			}
 
-			prg_mask = (Cart.prg_size/8/2) - 1;
+			prg_mask = (Cart.prg_size / 32) - 1;
 			chr_mask = (Cart.chr_size / 8) - 1;
 			SetMirrorType(Cart.pad_h, Cart.pad_v);