Calculated column displaying "#Name?" – SharePoint stack exchange

I try to display KPIs indicating how long an item in the list goes into each status.

Column Name = Time In 5.

= IF (OR (ISBLANK ([Date entered 6. ]), ISBLANK ([Date Entered 5. ])), "No date KPI", DATEDIF ([Date entered 6. ],[Date Entered 5.],"RE"))

Some items display an (expected) number, some display the "No KPI Date" message, but some display "#Name?". What would make it appear?