Fixed logical tie iteration edge-case.

abjad/tools/agenttools/IterationAgent.py

@@ -664,6 +664,7 @@ def by_logical_tie(
         prototype = scoretools.Leaf
         if pitched:
             prototype = (scoretools.Chord, scoretools.Note)
+        leaf, yielded = None, False
         if not reverse:
             for leaf in self.by_class(prototype):
                 yielded = False
@@ -681,7 +682,7 @@ def by_logical_tie(
                     if not nontrivial or not logical_tie.is_trivial:
                         yielded = True
                         yield logical_tie
-            if not yielded:
+            if leaf is not None and not yielded:
                 if tie_spanners and \
                     tuple(tie_spanners)[0]._is_my_first_leaf(leaf):
                     logical_tie = leaf._get_logical_tie()
@@ -711,7 +712,7 @@ def by_logical_tie(
                     if not nontrivial or not logical_tie.is_trivial:
                         yielded = True
                         yield logical_tie
-            if not yielded:
+            if leaf is not None and not yielded:
                 if tie_spanners and \
                     tuple(tie_spanners)[0]._is_my_last_leaf(leaf):
                     logical_tie = leaf._get_logical_tie()

abjad/tools/agenttools/test/test_agenttools_IterationAgent_by_logical_tie.py

@@ -143,3 +143,10 @@ def test_agenttools_IterationAgent_by_logical_tie_10():
     assert len(logical_ties[1]) == 1
     assert logical_ties[0][0] is staff[1][0]
     assert logical_ties[1][0] is staff[1][1]
+
+
+def test_agenttools_IterationAgent_by_logical_tie_11():
+    r'''No logical ties, but no errors either.'''
+    staff = Staff()
+    logical_ties = list(iterate(staff).by_logical_tie())
+    assert len(logical_ties) == 0