diff --git a/packages/logic-solver/logic.js b/packages/logic-solver/logic.js
index 337da15e12..a9f7581cd4 100644
--- a/packages/logic-solver/logic.js
+++ b/packages/logic-solver/logic.js
@@ -190,6 +190,8 @@ Meteor._inherits(Logic.OrFormula, Logic.Formula);
 Logic.OrFormula.prototype._genTrue = function (solver) {
   return [new Logic.Clause(solver._toN(this.terms))];
 };
-//Logic.OrFormula.prototype._genFalse = function (solver) {
-  // XXX
-//};
+Logic.OrFormula.prototype._genFalse = function (solver) {
+  return _.map(this.terms, function (t) {
+    return [new Logic.Clause(-solver._toN(t))];
+  });
+};
diff --git a/packages/logic-solver/logic_tests.js b/packages/logic-solver/logic_tests.js
index b801916f4c..c0f0d9a3cb 100644
--- a/packages/logic-solver/logic_tests.js
+++ b/packages/logic-solver/logic_tests.js
@@ -20,4 +20,6 @@ Tinytest.add("logic-solver - Logic.Or", function (test) {
   s.require(Logic.or('-c', 'd', 3));
   test.equal(s._clauseData(), [[3, 4], [-5, 6, 3]]);
 
+  s.forbid(Logic.or('a', '-b'));
+  test.equal(s._clauseData(), [[3, 4], [-5, 6, 3], [-3], [4]]);
 });