diff --git a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/Boards/ExROM.cs b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/Boards/ExROM.cs
index 2eb3fab5ab..9c609ee00d 100644
--- a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/Boards/ExROM.cs
+++ b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/Boards/ExROM.cs
@@ -1,3 +1,7 @@
+#if DEBUG
+using System.Diagnostics;
+#endif
+
 using BizHawk.Common;
 using BizHawk.Emulation.Common;
 using BizHawk.Emulation.Cores.Components;
@@ -169,7 +173,9 @@ namespace BizHawk.Emulation.Cores.Nintendo.NES
 		// for iNES, we assume 64K wram
 		private int? MaskWRAM(int bank)
 		{
-			bank &= 7;
+#if DEBUG
+			if (bank < 0 || (Cart.WramSize is 128 ? 0xF : 0x7) < bank) throw new ArgumentOutOfRangeException(paramName: nameof(bank), bank, message: "invalid bank index");
+#endif
 			switch (Cart.WramSize)
 			{
 				case 0:
@@ -521,7 +527,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.NES
 				
 
 				case 0x1113: //$5113:  [.... .PPP]        (simplified, but technically inaccurate -- see below)
-					wram_bank = value & 7;
+					wram_bank = value & (Cart.WramSize is 128 ? 0b1111 : 0b0111);
 					break;
 
 				//$5114-5117:  [RPPP PPPP] PRG select