A workflow or approval field update caused an error when saving this record. Contact your administrator to resolve it. Division undefined

When creating a workflow with field using values from formula fields I encountered this error: "A workflow or approval field update caused an error when saving this record. Contact your administrator to resolve it. Division undefined". I searched google and none was returned. My formula is:

(S1R__c * Points_Map__r.S1VP__c) +
(S2R__c * Points_Map__r.S2VP__c) +
(S3R__c * Points_Map__r.S3VP__c) +
(S4R__c * Points_Map__r.S4VP__c) +
(S5R__c * Points_Map__r.S5VP__c)

Where Points_Map__r is a lookup relationship within the same object. And all the involved fields are formula fields.

Turns out later that it is caused by a percentage formula (S5R) not getting a value.