I helped someone else out with this problem a few weeks ago.
This is direct from the factory: The original intention of Humminbird and Lakemaster was to have the 987 be capable of running Lakemaster chips through the standard software update that the rest of the units received. However, hardware limitations on the 987 prevented this from happening. As a result, the 987 does not support Lakemaster chips.
Text on both the Humminbird and Lakemaster websites needs to be updated to reflect this 987/Lakemaster compatibility issue.