Undefined kerning classes

I’m processing Glyphs file kerning rules from Babelfont, and found an interesting edge case that I’m not sure how to best handle. Nunito.glyphs has a glyph named “ze-cy”. At the moment, this glyph has rightKerningGroup = "ve-cy", although I’m sure at some point in the past it probably had rightKerningGroup = "ze-cy";.

I think this, because there is a kerning rule

"@MMK_L_Ge-cy" = {
"@MMK_R_ze-cy" = -80;

At this point no glyphs have rightKerningGroup = ze-cy, so this rule is meaningless. Babelfont gives an error Attempted to use undefined kerning class @MMK_R_ze-cy.

Glyphs does not complain about this. How should this be processed? Should it just ignore the kern rule? Should it warn? Should it error?