I posted this as a reply to old thread but noticed afterwards that it had been closed earlier.
As is after going through several topics under this forum it would seem that there is more widespread problem with the database containing historical stock quotes.
As an example I found errors for several companies listed in Helsinki Stock exchange such as Fortum (FORTUM.HE), Orion Oyj (ORNBV.HE) and Nokia (NOKIA.HE). Both of these companies have clearly null values in 2016 and prior dates. These values are listed as 0 on historical stock prices list and the error can be also seen when looking at the graph: https://finance.yahoo.com/quote/NOKIA.HE?p=NOKIA.HE
I believe this issue is somewhat wide spread as it came up with randomly checking out the historical stock prices of a few companies. Further looking at the graphs it would seem that majority of the companies in Helsinki have periods of missing data.