Column contains blank values and this is not allowed

Hey,

I got a question regarding relationships. When updating my dataset in the service I often get the following error:

"Column 'Column1' in Table 'Table1' contains blank values and this is not allowed for columns on the one side of a many-to-one relationship or for columns that are used as the primary key of a table."

I understand what this means, but I don't know why it happens. First of all there are safety measures in our database to prevent this from happening and secondly in my queries I filter out rows with blank values for any columns used in a relationship.

#"Removed empty Project IDs" = Table.SelectRows(#"Removed Other Columns", each [Column1] <> null and [Column1] <> ""),

So this error should not happen for two reasons.

Any idea what I am missing that could fix this problem?

Thank you.