diff --git a/src/main.cpp b/src/main.cpp
index b6888f975..ebb863396 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -2215,6 +2215,8 @@ void PushGetBlocks(CNode* pnode, CBlockIndex* pindexBegin, uint256 hashEnd)
 
 bool ProcessBlock(CValidationState &state, CNode* pfrom, CBlock* pblock, CDiskBlockPos *dbp)
 {
+    AssertLockHeld("cs_main");
+
     // Check for duplicate
     uint256 hash = pblock->GetHash();
     if (mapBlockIndex.count(hash))
diff --git a/src/sync.cpp b/src/sync.cpp
index 33e121954..9a20c87f8 100644
--- a/src/sync.cpp
+++ b/src/sync.cpp
@@ -42,6 +42,8 @@ struct CLockLocation
         return mutexName+"  "+sourceFile+":"+itostr(sourceLine);
     }
 
+    std::string MutexName() const { return mutexName; }
+
 private:
     std::string mutexName;
     std::string sourceFile;
@@ -126,4 +128,20 @@ void LeaveCritical()
     pop_lock();
 }
 
+std::string LocksHeld()
+{
+    std::string result;
+    BOOST_FOREACH(const PAIRTYPE(void*, CLockLocation)&i, *lockstack)
+        result += i.second.ToString() + std::string("\n");
+    return result;
+}
+
+void AssertLockHeld(std::string strName)
+{
+    BOOST_FOREACH(const PAIRTYPE(void*, CLockLocation)&i, *lockstack)
+        if (i.second.MutexName() == strName) return;
+    LogPrintf("Lock %s not held; locks held:\n%s", strName.c_str(), LocksHeld().c_str());
+    assert(0);
+}
+
 #endif /* DEBUG_LOCKORDER */
diff --git a/src/sync.h b/src/sync.h
index 39f2cb515..c50abf81b 100644
--- a/src/sync.h
+++ b/src/sync.h
@@ -87,9 +87,12 @@ typedef AnnotatedMixin<boost::mutex> CWaitableCriticalSection;
 #ifdef DEBUG_LOCKORDER
 void EnterCritical(const char* pszName, const char* pszFile, int nLine, void* cs, bool fTry = false);
 void LeaveCritical();
+std::string LocksHeld();
+void AssertLockHeld(std::string strName);
 #else
 void static inline EnterCritical(const char* pszName, const char* pszFile, int nLine, void* cs, bool fTry = false) {}
 void static inline LeaveCritical() {}
+void static inline AssertLockHeld(std::string) {}
 #endif
 
 #ifdef DEBUG_LOCKCONTENTION