Guided Rule Editor: Too much recursion when using an expression

Description of problem:

Version-Release number of selected component (if applicable):
6.1 ER3

Steps to Reproduce:
1. Create POJO Fact1 with String field1.
2. Create a guided rule, add Fact1 to when section, bind it to variable f1.
3. Add another Fact1 to when section, bind it to variable f2.
4. Add constraint on field1 to f1, operator 'is not null'.

• f1: Fact1(field1 != null)
  5. Add constraint on field1 to f2, operator 'is equal to', value f1.field1 built via expression editor.
• f2: Fact1(field1 == f1.field1)
  6. Validate the rule.

Actual results:
Error pop-up: "Unable to complete your request. The following exception occurred: (InternalError) stack: MJg@http://localhost:8080/ ..... columnNumber: 0: too much recursion."
Nothing special in server log.

Expected results:
Rule passes validation.

Additional info:
This rule cannot be saved. Clicking save (in effect) does nothing.