diff --git a/src/BizHawk.Emulation.Cores/Consoles/Sega/gpgx64/GPGX.cs b/src/BizHawk.Emulation.Cores/Consoles/Sega/gpgx64/GPGX.cs
index 80206da5d9..a42f6c86ad 100644
--- a/src/BizHawk.Emulation.Cores/Consoles/Sega/gpgx64/GPGX.cs
+++ b/src/BizHawk.Emulation.Cores/Consoles/Sega/gpgx64/GPGX.cs
@@ -39,7 +39,8 @@ namespace BizHawk.Emulation.Cores.Consoles.Sega.gpgx
 
 			// internally, the core caches disc sectors read into the invisible heap
 			var discBufferSize = lp.Discs.Sum(d => d.DiscData.Session1.LeadoutLBA * (2352 + 96 + 2));
-			discBufferSize += 1024 - discBufferSize % 1024;
+			discBufferSize /= 1024;
+			discBufferSize++;
 
 			_elf = new WaterboxHost(new WaterboxOptions
 			{