diff --git a/src/BizHawk.Emulation.Common/Database/FirmwareDatabase.cs b/src/BizHawk.Emulation.Common/Database/FirmwareDatabase.cs
index f385711397..ddb80af2da 100644
--- a/src/BizHawk.Emulation.Common/Database/FirmwareDatabase.cs
+++ b/src/BizHawk.Emulation.Common/Database/FirmwareDatabase.cs
@@ -47,8 +47,6 @@ namespace BizHawk.Emulation.Common
 						additionalInfo: additionalInfo,
 						isBad: isBad);
 
-			// make sure id doesn't have a space, it is stored in the (space delimited) movie header!
-
 			void Option(string systemId, string id, in FirmwareFile ff, FirmwareOptionStatus status = FirmwareOptionStatus.Acceptable)
 			{
 				var option = new FirmwareOption(new(systemId, id), ff.Hash, ff.Size, ff.IsBad ? FirmwareOptionStatus.Bad : status);
diff --git a/src/BizHawk.Emulation.Common/Database/FirmwareID.cs b/src/BizHawk.Emulation.Common/Database/FirmwareID.cs
index fb259f4904..70173dc9c4 100644
--- a/src/BizHawk.Emulation.Common/Database/FirmwareID.cs
+++ b/src/BizHawk.Emulation.Common/Database/FirmwareID.cs
@@ -1,3 +1,6 @@
+using System;
+using System.Linq;
+
 namespace BizHawk.Emulation.Common
 {
 	public readonly struct FirmwareID
@@ -16,6 +19,11 @@ namespace BizHawk.Emulation.Common
 
 		public FirmwareID(string system, string firmware)
 		{
+			static bool IsAllowedCharacter(char c)
+				=> c is '-' or (>= '0' and <= '9') or (>= 'A' and <= 'Z') or '_' or (>= 'a' and <= 'z');
+			const string ERR_MSG_INVALID_CHAR = "FWIDs must match /[-0-9A-Z_a-z]+/";
+			if (!system.All(IsAllowedCharacter)) throw new ArgumentOutOfRangeException(paramName: system, actualValue: system, message: ERR_MSG_INVALID_CHAR);
+			if (!firmware.All(IsAllowedCharacter)) throw new ArgumentOutOfRangeException(paramName: firmware, actualValue: firmware, message: ERR_MSG_INVALID_CHAR);
 			System = system;
 			Firmware = firmware;
 		}