Error updating existing record in Acumatica using API PUT

I’m trying to update an existing stock item record using a PUT request:

but I’m getting the following "Object reference not set to an instance of an object." 500 error from the server. Why is this happening? I can retrieve the record just fine using a GET request.

{
    "message": "An error has occurred.",
    "exceptionMessage": "Operation failed",
    "exceptionType": "PX.Data.PXInvalidOperationException",
    "stackTrace": "   at PX.Api.ContractBased.EntityService.Put(ISystemContract systemContract, String version, String name, EntityImpl entity, CbOperationContext operationContext, Boolean throwOnError)\r\n   at PX.Api.ContractBased.Soap.SoapFacadeBase.PutImpl(EntityImpl entity, Boolean throwOnValidationError)\r\n   at PX.Api.ContractBased.SystemContracts.V2.RestController.PutEntity(EntityImpl entity, String select, String filter, String expand, String custom)\r\n   at lambda_method(Closure , Object , Object[] )\r\n   at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.<>c__DisplayClass6_2.<GetExecutor>b__2(Object instance, Object[] methodParameters)\r\n   at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ExecuteAsync(HttpControllerContext controllerContext, IDictionary`2 arguments, CancellationToken cancellationToken)\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Controllers.ApiControllerActionInvoker.<InvokeActionAsyncCore>d__1.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__6.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__6.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Filters.ActionFilterAttribute.<ExecuteActionFilterAsyncCore>d__5.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__6.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__6.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Filters.ActionFilterAttribute.<ExecuteActionFilterAsyncCore>d__5.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Controllers.ActionFilterResult.<ExecuteAsync>d__5.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Filters.AuthorizationFilterAttribute.<ExecuteAuthorizationFilterAsyncCore>d__3.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Controllers.ExceptionFilterResult.<ExecuteAsync>d__6.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Web.Http.Controllers.ExceptionFilterResult.<ExecuteAsync>d__6.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Dispatcher.HttpControllerDispatcher.<SendAsync>d__15.MoveNext()",
    "innerException": {
        "message": "An error has occurred.",
        "exceptionMessage": "Object reference not set to an instance of an object.",
        "exceptionType": "System.NullReferenceException",
        "stackTrace": "   at KN.CP.KNCPInventoryItemMaintExt.InventoryItem_RowSelected(PXCache cache, PXRowSelectedEventArgs e, PXRowSelected BaseSelectedevent)\r\n   at KN.DistrictCameras.KNDCInventoryItemMaintBaseExt.InventoryItem_RowSelected(PXCache cache, PXRowSelectedEventArgs e, PXRowSelected BaseEvent)\r\n   at KN.S3Images.KNCPInventoryItemMaintS3Ext.InventoryItem_RowSelected(PXCache cache, PXRowSelectedEventArgs e, PXRowSelected BaseSelectedevent)\r\n   at KNCommerceBasic.KNCBInventoryItemMaintExt.InventoryItem_RowSelected(PXCache cache, PXRowSelectedEventArgs e, PXRowSelected BaseEvent)\r\n   at KNMagentoConnector.KNMCInventoryItemMaintExtn.InventoryItem_RowSelected(PXCache cache, PXRowSelectedEventArgs e, PXRowSelected BaseEvent)\r\n   at PX.Data.PXRowSelected.Invoke(PXCache sender, PXRowSelectedEventArgs e)\r\n   at PX.Data.PXCache.OnRowSelected(Object item)\r\n   at PX.Api.SyImportProcessor.SyStep.CommitChangesInt(Object itemToBypass, PXFilterRow[] targetConditions, PXFilterRow[] filtersForAction, SyImportRowResult importResult)\r\n   at PX.Api.SyImportProcessor.SyStep.CommitChanges(Object& itemToBypass, PXFilterRow[] targetConditions, PXFilterRow[] filtersForAction)\r\n   at PX.Api.SyImportProcessor.ExportTableHelper.ExportTable()"
    }
}