File not found error from dotnet-svcutil

I have an old WCF service, and just finished porting it to .NET 6 (using CoreWCF). The service seems to run fine and I can retrieve metadata over an http connection. I have been trying to use dotnet-svcutil 2.1.0 to update the service reference but have run into a few problems. I worked around most of them, but I have not been able to find a way around this one...

Microsoft (R) WCF Service Model Proxy Generation Tool for .Net Core platform
[Microsoft.Tools.ServiceModel.Svcutil, Version 2.1.0]
Copyright (c) Microsoft Corporation. All rights reserved.

This tool collects information about how it is used in order to improve the tool. This functionality can be disabled by setting the environment variable "DOTNET_SVCUTIL_TELEMETRY_OPTOUT" to 1.

Resolving project references ...
Bootstrapping application ...
Attempting to download metadata from '[http://localhost:8020/EquipmentControlServer/mex?wsdl'](http://localhost:8020/EquipmentControlServer/mex?wsdl%27) using WS-Metadata Exchange and HttpGet.
Warning: Cannot import wsdl:portType
Detail: An exception was thrown while running a WSDL import extension: System.ServiceModel.Description.DataContractSerializerMessageContractImporter
Error: Could not load file or assembly 'System.Windows.Forms, Version=6.0.2.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'. The system cannot find the file specified.

I don't think there's anything in the service contract that requires this reference (I don't see anything in the WSDL file) but I suppose it's possible. I can't find this particular version of System.Windows.Forms on my computer (but there are many versions, and I haven't checked them all). I suspect it is the bootstrapper that requires this reference, but I don't know how to confirm that.

Has anyone else experience something like this?