Spring element between two RP

Guys, I've got two parts and want to connect them together using a spring element. I've done following:
1- Create two RP for the two parts
2- Couple each RP to its relevant part
3- Connecting two RP using an spring element
when I run the model using explicit solver I get following error:

"Spring elements in element set ErrElemSpringsWithoutMass-Step1 are connected to massless nodes."

Does anyone knows what is the problem?

 

[Me09]

so, in explicit, if I want to connect a spring between two reference points which are coupled with two bodies, do i have to define mass for the reference points?

 

[Me09]

is there a tip to get rid of mass definition for the RPs?
obviously I don't want to connect the the spring directly to a node on the body as I believe it causes local effect on the body. is it right?