ContentImporter error at Microsoft.Xna.Framework.Content.Pipeline.Graphics.IndexCollection.AddRang - any ideas

Hi - im getting an occasional build error when importing an FBX model.

I have many working models. It seems to occur based on the material of the model. When I change the material, the model imports successfully. Does anyone have a clue? Or perhaps can someone show me where the source code is? Even some better error handling would be great.

Thoughts welcome

error : Importer 'FbxImporter' had unexpected failure!
1>System.NullReferenceException: Object reference not set to an instance of an object.
1>   at Microsoft.Xna.Framework.Content.Pipeline.Graphics.IndexCollection.AddRange(IEnumerable`1 indices) in /home/runner/work/MonoGame/MonoGame/MonoGame.Framework.Content.Pipeline/Graphics/IndexCollection.cs:line 28
1>   at Microsoft.Xna.Framework.Content.Pipeline.OpenAssetImporter.CreateGeometry(MeshContent mesh, Mesh aiMesh) in /home/runner/work/MonoGame/MonoGame/MonoGame.Framework.Content.Pipeline/OpenAssetImporter.cs:line 619
1>   at Microsoft.Xna.Framework.Content.Pipeline.OpenAssetImporter.ImportNodes(Node aiNode, Node aiParent, NodeContent parent) in /home/runner/work/MonoGame/MonoGame/MonoGame.Framework.Content.Pipeline/OpenAssetImporter.cs:line 525
1>   at Microsoft.Xna.Framework.Content.Pipeline.OpenAssetImporter.ImportNodes(Node aiNode, Node aiParent, NodeContent parent) in /home/runner/work/MonoGame/MonoGame/MonoGame.Framework.Content.Pipeline/OpenAssetImporter.cs:line 612
1>   at Microsoft.Xna.Framework.Content.Pipeline.OpenAssetImporter.ImportNodes() in /home/runner/work/MonoGame/MonoGame/MonoGame.Framework.Content.Pipeline/OpenAssetImporter.cs:line 491
1>   at Microsoft.Xna.Framework.Content.Pipeline.OpenAssetImporter.Import(String filename, ContentImporterContext context) in /home/runner/work/MonoGame/MonoGame/MonoGame.Framework.Content.Pipeline/OpenAssetImporter.cs:line 330
1>   at Microsoft.Xna.Framework.Content.Pipeline.FbxImporter.Import(String filename, ContentImporterContext context) in /home/runner/work/MonoGame/MonoGame/MonoGame.Framework.Content.Pipeline/FbxImporter.cs:line 24
1>   at Microsoft.Xna.Framework.Content.Pipeline.ContentImporter`1.Microsoft.Xna.Framework.Content.Pipeline.IContentImporter.Import(String filename, ContentImporterContext context) in /home/runner/work/MonoGame/MonoGame/MonoGame.Framework.Content.Pipeline/ContentImporter.cs:line 45
1>   at MonoGame.Framework.Content.Pipeline.Builder.PipelineManager.ProcessContent(PipelineBuildEvent pipelineEvent) in /home/runner/work/MonoGame/MonoGame/MonoGame.Framework.Content.Pipeline/Builder/PipelineManager.cs:line 669