diff --git a/re2/dfa.cc b/re2/dfa.cc
index e1b897c..27fd049 100644
--- a/re2/dfa.cc
+++ b/re2/dfa.cc
@@ -1955,7 +1955,7 @@
 }
 
 // Build out all states in DFA for kind.  Returns number of states.
-int Prog::BuildEntireDFA(MatchKind kind, DFAStateCallback cb) {
+int Prog::BuildEntireDFA(MatchKind kind, const DFAStateCallback& cb) {
   return GetDFA(kind)->BuildAllStates(cb);
 }
 
diff --git a/re2/prog.h b/re2/prog.h
index fa24341..3176898 100644
--- a/re2/prog.h
+++ b/re2/prog.h
@@ -272,7 +272,7 @@
   // If cb is not empty, it receives one callback per state built.
   // Returns the number of states built.
   // FOR TESTING OR EXPERIMENTAL PURPOSES ONLY.
-  int BuildEntireDFA(MatchKind kind, DFAStateCallback cb);
+  int BuildEntireDFA(MatchKind kind, const DFAStateCallback& cb);
 
   // Controls whether the DFA should bail out early if the NFA would be faster.
   // FOR TESTING ONLY.