diff --git a/src/BizHawk.Emulation.Common/Database/FirmwareDatabase.cs b/src/BizHawk.Emulation.Common/Database/FirmwareDatabase.cs
index baa70fc4f1..e50098fb59 100644
--- a/src/BizHawk.Emulation.Common/Database/FirmwareDatabase.cs
+++ b/src/BizHawk.Emulation.Common/Database/FirmwareDatabase.cs
@@ -39,13 +39,16 @@ namespace BizHawk.Emulation.Common
 				string desc,
 				string additionalInfo = "",
 				bool isBad = false)
-					=> filesByHash[hash] = new(
-						hash: hash,
-						size: size,
-						recommendedName: recommendedName,
-						desc: desc,
-						additionalInfo: additionalInfo,
-						isBad: isBad);
+			{
+				FirmwareFile ff = new(
+					hash: hash,
+					size: size,
+					recommendedName: recommendedName,
+					desc: desc,
+					additionalInfo: additionalInfo,
+					isBad: isBad);
+				return filesByHash[ff.Hash/*may have been reformatted*/] = ff;
+			}
 
 			void Option(string systemId, string id, in FirmwareFile ff, FirmwareOptionStatus status = FirmwareOptionStatus.Acceptable)
 			{
diff --git a/src/BizHawk.Emulation.Common/Database/FirmwareFile.cs b/src/BizHawk.Emulation.Common/Database/FirmwareFile.cs
index a855962b5c..4dd59396ec 100644
--- a/src/BizHawk.Emulation.Common/Database/FirmwareFile.cs
+++ b/src/BizHawk.Emulation.Common/Database/FirmwareFile.cs
@@ -1,7 +1,24 @@
+using System;
+using System.Linq;
+
+using BizHawk.Common.StringExtensions;
+
 namespace BizHawk.Emulation.Common
 {
 	public readonly struct FirmwareFile
 	{
+		internal static void CheckChecksumStrIsHex(ref string digest)
+		{
+			static bool IsAllowedCharacter(char c)
+				=> c is (>= '0' and <= '9') or (>= 'A' and <= 'F'); //TODO SearchValues?
+			if (digest.All(IsAllowedCharacter)) return;
+			if (!digest.IsHex()) throw new ArgumentOutOfRangeException(paramName: nameof(digest), actualValue: digest, message: "malformed checksum digest: must match /[0-9A-F]+/ (no lowercase please)");
+			// but if it is hex, let's be lenient
+			Console.Write("interpreting as hex checksum digest and uppercasing (please fix in source): ");
+			Console.WriteLine(digest);
+			digest = digest.ToUpperInvariant();
+		}
+
 		public readonly string Description;
 
 		public readonly string Hash;
@@ -22,6 +39,7 @@ namespace BizHawk.Emulation.Common
 			string additionalInfo = "",
 			bool isBad = false)
 		{
+			CheckChecksumStrIsHex(ref hash);
 			Description = desc;
 			Hash = hash;
 			Info = additionalInfo;
diff --git a/src/BizHawk.Emulation.Common/Database/FirmwareOption.cs b/src/BizHawk.Emulation.Common/Database/FirmwareOption.cs
index fa5ec1dc1e..3fb72926b6 100644
--- a/src/BizHawk.Emulation.Common/Database/FirmwareOption.cs
+++ b/src/BizHawk.Emulation.Common/Database/FirmwareOption.cs
@@ -22,6 +22,7 @@ namespace BizHawk.Emulation.Common
 
 		public FirmwareOption(FirmwareID id, string hash, long size, FirmwareOptionStatus status)
 		{
+			FirmwareFile.CheckChecksumStrIsHex(ref hash);
 			Hash = hash;
 			ID = id;
 			Size = size;
diff --git a/src/BizHawk.Emulation.Common/Database/FirmwarePatchOption.cs b/src/BizHawk.Emulation.Common/Database/FirmwarePatchOption.cs
index 6cd4d69876..66197c0a44 100644
--- a/src/BizHawk.Emulation.Common/Database/FirmwarePatchOption.cs
+++ b/src/BizHawk.Emulation.Common/Database/FirmwarePatchOption.cs
@@ -15,6 +15,8 @@ namespace BizHawk.Emulation.Common
 
 		public FirmwarePatchOption(string baseHash, IReadOnlyList<FirmwarePatchData> patches, string targetHash)
 		{
+			FirmwareFile.CheckChecksumStrIsHex(ref baseHash);
+			FirmwareFile.CheckChecksumStrIsHex(ref targetHash);
 			BaseHash = baseHash;
 			Patches = patches;
 			TargetHash = targetHash;