Polyline Edit Error Message

The error message:

An entity which is defined in the current UCS is needed

...can sometimes pop up when you are running the Polyline Edit command.  This message can be misleading.

It usually indicates that you have selected an entity for the Polyline Edit command that is not "flat."  In other words the entity may be a line or 3D polyline which has a different Z value at one end compared with the other.

The Polyline Edit command is very handy for combining adjacent entities, but be warned, it will convert every entity into a 2D polyline with one Z value from start to end.