diff --git a/src/rpcblockchain.cpp b/src/rpcblockchain.cpp
index 39f62dfa1..4915bd0cf 100644
--- a/src/rpcblockchain.cpp
+++ b/src/rpcblockchain.cpp
@@ -1419,7 +1419,7 @@ UniValue getchaintips(const UniValue& params, bool fHelp)
        of another block.  */
     static pthread_mutex_t mutex; static int32_t didinit;
     if ( didinit == 0 )
-        pthread_mutex_init(&mutex);
+        pthread_mutex_init(&mutex,NULL);
     pthread_mutex_lock(&mutex);
     std::set<const CBlockIndex*, CompareBlocksByHeight> setTips;
     int32_t n = 0;