diff --git a/README.md b/README.md
index 4a12b6f..8b970a1 100644
--- a/README.md
+++ b/README.md
@@ -105,7 +105,7 @@ In case of failure, ``nil`` is returned.
 - ``applicable?(precond_pos, precond_not)`` tests if the current state have all positive preconditions and not a single negative precondition. Returns ``true`` if applicable, ``false`` otherwise.
 - ``apply(effect_add, effect_del)`` modifies the current state, add or remove predicates present in the lists. Returns ``true``.
 - ``apply_operator(precond_pos, precond_not, effect_add, effect_del)`` applies effects if ``applicable?``. Returns ``true`` if applied, ``nil`` otherwise.
-- ``generate(precond_pos, precond_not, *free)`` yields all possible unifications to the free variables defined, therefore a block is needed to capture the unifications. Return value is undefined.
+- ``generate(precond_pos, precond_not, *free)`` yields all possible unifications to the free variables defined, therefore a block is needed to capture the unifications. Returns ``nil``.
 - ``print_data(data)`` can be used to print task and predicate lists, useful for debug.
 - ``problem(state, tasks, debug = false, ordered = true)`` simplifies the setup of a problem instance, returns the value of planning. Use problem as a template to see how to add HyperTensioN in a project.
 - ``task_permutations(state, tasks, goal_task = nil)`` tries permutations of ``tasks`` to achieve unordered decomposition, ``goal_task`` is the final task, used by ``problem``. Returns a plan or ``nil``.
diff --git a/tests/sphygmomanometer.rb b/tests/sphygmomanometer.rb
index 751768a..174ef3a 100644
--- a/tests/sphygmomanometer.rb
+++ b/tests/sphygmomanometer.rb
@@ -75,7 +75,7 @@ def test_generate
     w = ''
     z = ''
     # Generate x y w z based on state and preconditions
-    Hypertension.generate(
+    assert_nil(Hypertension.generate(
       [
         [A, x],
         [B, y],
@@ -87,7 +87,7 @@ def test_generate
       ], x, y, w, z
     ) {
       assert_equal(expected.shift, [x,y,w,z])
-    }
+    })
     assert_true(expected.empty?)
   end
 
@@ -101,7 +101,7 @@ def test_generate_exist
     z = ''
     # Generate x y w z based on state and preconditions
     exist_y = nil # One unification of y is enough
-    Hypertension.generate(
+    assert_nil(Hypertension.generate(
       [
         [A, x],
         [B, y],
@@ -114,7 +114,7 @@ def test_generate_exist
     ) {
       break if (exist_y ||= y.dup) != y
       assert_equal(expected.shift, [x,y,w,z])
-    }
+    })
     assert_true(expected.empty?)
   end