The Equation constraint in Abaqus

I have a question about the EQUATION constraint to apply periodical boundary conditions via Script in Abaqus. The detailes is as follows

(1) Create the node sets firstly in the PART Module;

(2) Using script to create node sets containing only one node;

(3) Using the following script to establish the EQUATION constraint

mdb.models['Job-1'].Equation(name=’Constraint-1′, terms=((1.0,

‘PART-1-1.Node-1′, 1), (1.0, ‘PART-1-1.Node-2′, 1), (-1.0, ‘RP1′, 1)))

(Two nodes ‘PART-1-1.Node-1’and ‘PART-1-1.Node-2’are created during the second step.RP1 is the reference point defined in the Abaqus/CAE)

ValueError: Set PART-1-1.Node-1 does not exist.

Set PART-1-1.Node-2 does not exist.

please help me to solve this question, many thanks