diff --git a/src/bees-trace.cc b/src/bees-trace.cc
index 5e58532..81b70d5 100644
--- a/src/bees-trace.cc
+++ b/src/bees-trace.cc
@@ -49,7 +49,7 @@ BeesTracer::~BeesTracer()
 	}
 }
 
-BeesTracer::BeesTracer(function<void()> f, bool silent) :
+BeesTracer::BeesTracer(const function<void()> &f, bool silent) :
 	m_func(f)
 {
 	m_next_tracer = tl_next_tracer;
diff --git a/src/bees.h b/src/bees.h
index 10ed7f6..888f91e 100644
--- a/src/bees.h
+++ b/src/bees.h
@@ -193,7 +193,7 @@ class BeesTracer {
 	thread_local static bool tl_silent;
 	thread_local static bool tl_first;
 public:
-	BeesTracer(function<void()> f, bool silent = false);
+	BeesTracer(const function<void()> &f, bool silent = false);
 	~BeesTracer();
 	static void trace_now();
 	static bool get_silent();