SPULSRetriever.GetULSEntries() throws exception

I've been trying to grab the ULS log entries as part of exception handling in a page to store the log text so a user can read it without having to get access to the server. I've tried using elevated privileges. This is on SharePoint 2013 on an application page.

No matter what, I get an exception:

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object. at Microsoft.SharePoint.Diagnostics.SPUsageDBScopeStore.ReadScopesForCorrelationId(Guid
 correlationId) at Microsoft.SharePoint.Diagnostics.SPUsageDBScopeStore.<>c__DisplayClass7.<Get>b__6() at Microsoft.SharePoint.Utilities.SecurityContext.RunAsProcess(CodeToRunElevated secureCode) at Microsoft.SharePoint.Diagnostics.SPUsageDBScopeStore.Get(Guid
 correlationId) at Microsoft.SharePoint.Diagnostics.SPULSRetriever.GetULSEntries(Guid correlationId) at MyPage.<>c__DisplayClass24.<GetULSInfo>b__21() at Microsoft.SharePoint.SPSecurity.<>c__DisplayClass5.<RunWithElevatedPrivileges>b__3()
 at Microsoft.SharePoint.Utilities.SecurityContext.RunAsProcess(CodeToRunElevated secureCode) at Microsoft.SharePoint.SPSecurity.RunWithElevatedPrivileges(WaitCallback secureCode, Object param) at Microsoft.SharePoint.SPSecurity.RunWithElevatedPrivileges(CodeToRunElevated
 secureCode) at MyPage.GetULSInfo()

Test code is this:

private string GetULSInfo()
        {
            string ulsLog = string.Empty;
            try
            {
                Logger.LogInfoToULS("Testing info log");
                Logger.LogErrorToULS("Testing Error log");
                var guid = new Guid();
                EventActivityIdControl(1, ref guid);
                Logger.LogInfoToULS("Getting log for correlation id " + guid.ToString());
                SPSecurity.RunWithElevatedPrivileges(
                            delegate()
                            {
                                SPULSRetriever ulsRetriever = new SPULSRetriever(20, 200, DateTime.Now.AddMinutes(-10));

                                IList<LogFileEntry> ulsLogEntries = ulsRetriever.GetULSEntries(guid);
                                if (ulsLogEntries.Count > 0)
                                {
                                    StringBuilder sb = new StringBuilder();
                                    foreach (LogFileEntry logEntry in ulsLogEntries)
                                    {
                                        if (logEntry.Category.StartsWith("EPS", StringComparison.InvariantCultureIgnoreCase))
                                        {
                                            sb.AppendLine(logEntry.Message);
                                        }
                                    }
                                    ulsLog = sb.ToString();
                                }
                            });
            }
            catch (Exception ex)
            {
                Logger.LogErrorToULS("OperationManager::GetULSInfo", ex);
            }
            return ulsLog;

It throws the exception at the GetULSEntries(guid) line. Nothing shows up in system logs.

Can anyone shed light on this? Very sparse info found with internet searches.