Error using Inventory Set On Hand Quantities mutation

I am using Workato to develop integration between our Shopify website and our ERP system. Workato has a GraphQL connector that adapts to the API it’s connected to and it seems to provide all the configuration options for the InventorySetOnHandQuantities mutation, but when I test the mutation, I get an error -

Field must have selections (field ‘inventorySetOnHandQuantities’ returns InventorySetOnHandQuantitiesPayload but has no selections. Did you mean ‘inventorySetOnHandQuantities { … }’?) at line: 1328 called from: 1340 called from: 1324 called from: 2780 called from: 677

I’ve compared my payload with the example on the API reference page, and there is nothing that jumps out at me as an obvious problem with the request.> > Any suggestions on correcting this error? Once I had the Inventory ID for the part variation and the location ID, I did confirm that I could use the REST API to change the quantity, so it isn’t a permission issue. I don’t want to have to use both GraphQL and REST because that would cost me an additional monthly connector fee. I’d just use REST - but I haven’t been able to figure out a way to use my SKU to get the inventory ID that I need to change the inventory quantity.