diff --git a/spacy/syntax/transition_system.pyx b/spacy/syntax/transition_system.pyx
index 09477f8e0..d88ab26bd 100644
--- a/spacy/syntax/transition_system.pyx
+++ b/spacy/syntax/transition_system.pyx
@@ -87,9 +87,9 @@ cdef class TransitionSystem:
                 if is_valid[i] and costs[i] <= 0:
                     action = self.c[i]
                     history.append(i)
-                    s0 = state.S(0)
-                    b0 = state.B(0)
                     if _debug:
+                        s0 = state.S(0)
+                        b0 = state.B(0)
                         example = _debug
                         debug_log.append(" ".join((
                             self.get_class_name(i),