Using DISTINCT with Ordered Analytic Functions and Dates

Database
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Using DISTINCT with Ordered Analytic Functions and Dates

I have run into a perplexing little problem. I know how to work around it, but want to understand the reason it is occuring.

When I use the FIRST_VALUE function on a date as below, it returns two rows:

SELECT  
l.Id ID,
FIRST_VALUE(t.LASTMODIFIEDDATE) OVER(PARTITION BY l.ID ORDER BY t.LASTMODIFIEDDATE) First_Contacted


I only want to return a single row, so I add DISTINCT to the code:

SELECT DISTINCT
l.Id ID,
FIRST_VALUE(t.LASTMODIFIEDDATE) OVER(PARTITION BY l.ID ORDER BY t.LASTMODIFIEDDATE) First_Contacted

This returns the error: Failed [5407 : HY000] Invalid operation for DateTime or Interval. 

Why????