I'm getting the following error in a 4.2 upgraded to 5.1.2 instance

Unhandled exception: The INSERT statement conflicted with the FOREIGN KEY constraint "geminiwatchissuesuseridfk". The conflict occurred in database "Gemini", table "dbo.geminiusers", column 'userid'. The statement has been terminated.[SQL: INSERT INTO geminiwatchissues (issueid, projectid, userid) VALUES (?, ?, ?); select SCOPEIDENTITY()]. Stack trace: at NHibernate.Id.Insert.AbstractReturningDelegate.PerformInsert(SqlCommandInfo insertSQL, ISessionImplementor session, IBinder binder) at NHibernate.Persister.Entity.AbstractEntityPersister.Insert(Object[] fields, Object obj, ISessionImplementor session) at NHibernate.Action.EntityIdentityInsertAction.Execute() at NHibernate.Engine.ActionQueue.Execute(IExecutable executable) at NHibernate.Event.Default.AbstractSaveEventListener.PerformSaveOrReplicate(Object entity, EntityKey key, IEntityPersister persister, Boolean useIdentityColumn, Object anything, IEventSource source, Boolean requiresImmediateIdAccess) at NHibernate.Event.Default.AbstractSaveEventListener.SaveWithGeneratedId(Object entity, String entityName, Object anything, IEventSource source, Boolean requiresImmediateIdAccess) at NHibernate.Event.Default.DefaultSaveOrUpdateEventListener.SaveWithGeneratedOrRequestedId(SaveOrUpdateEvent event) at NHibernate.Event.Default.DefaultSaveOrUpdateEventListener.EntityIsTransient(SaveOrUpdateEvent event) at NHibernate.Event.Default.DefaultSaveOrUpdateEventListener.OnSaveOrUpdate(SaveOrUpdateEvent event) at NHibernate.Impl.SessionImpl.FireSave(SaveOrUpdateEvent event) at NHibernate.Impl.SessionImpl.Save(Object obj) at Countersoft.Gemini.Data.Issues.IssueRepository.UpdateWatchers(Int32 issueId, Int32 projectId, List1 watchers) at Countersoft.Gemini.Business.Issues.Update(Issue entity) at Countersoft.Gemini.Infrastructure.Managers.IssueManager.Update(Issue entity) at Countersoft.Gemini.Infrastructure.Managers.IssueManager.IssueCommentCreate(IssueComment comment) at Countersoft.Gemini.Controllers.AjaxItemController.AddComment(Int32 projectId, Int32 issueId) at lambda_method(Closure , ControllerBase , Object[] ) at System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary2 parameters) at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>cDisplayClass37.<>cDisplayClass39.b33() at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>cDisplayClass4f.b49() at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>cDisplayClass4f.b49() at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>cDisplayClass37.b36(IAsyncResult asyncResult) at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>cDisplayClass25.<>cDisplayClass2a.b20() at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>cDisplayClass25.b22(IAsyncResult asyncResult) at System.Web.Mvc.Controller.<>cDisplayClass1d.b18(IAsyncResult asyncResult) at System.Web.Mvc.Async.AsyncResultWrapper.<>cDisplayClass4.b3(IAsyncResult ar) at System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) at System.Web.Mvc.Async.AsyncResultWrapper.<>cDisplayClass4.b3(IAsyncResult ar) at System.Web.Mvc.MvcHandler.<>cDisplayClass8.b3(IAsyncResult asyncResult) at System.Web.Mvc.Async.AsyncResultWrapper.<>cDisplayClass4.b3(IAsyncResult ar) at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)