mongodb the connection has been terminated, and this cursor is dead

mongodb the connection has been terminated, and this cursor is dead

I want to populate a mongodb database with data using doctrine and
zend-framework2. All works fine until the number of records in a
collection grows to about 2 million and then I get the following
exception: "the connection has been terminated, and this cursor is dead"
Please, can anybody tell me why this exception is thrown and how to
resolve this? Thank you all!