Blank, nullable date column rejected by psql

Running postgresql-10.5 on Slackware-14.2.

A table has a column defined as

Column       | Type  | Collation | Nullable | Default
next_contact | date  |           |          |

In a .sql file to insert rows in this table psql has a problem when there's
no value for the next_contact column:

$ psql -f activities.sql -d bustrac 
psql:activities.sql:6: ERROR:  invalid input syntax for type date: ""
LINE 2: ...ise. Asked him to call.',''),

Explicitly replacing the blank field ('') with null is accepted. Why is
this?

Now I know to replace no dates with null I'll do so but I'm curious why this
is needed.

Thanks in advance,

Rich