diff --git a/src/init.cpp b/src/init.cpp
index 02dca57031..ac1b68de8b 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -1396,7 +1396,7 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
     // Monitor the chain, and alert if we get blocks much quicker or slower than expected
     int64_t nPowTargetSpacing = Params().GetConsensus().nPowTargetSpacing;
     CScheduler::Function f = boost::bind(&PartitionCheck, &IsInitialBlockDownload,
-                                         boost::ref(cs_main), boost::cref(chainActive), nPowTargetSpacing);
+                                         boost::ref(cs_main), boost::cref(pindexBestHeader), nPowTargetSpacing);
     scheduler.scheduleEvery(f, nPowTargetSpacing);
 
 #ifdef ENABLE_WALLET
diff --git a/src/main.cpp b/src/main.cpp
index e9a5f7efd9..d74db2b920 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1711,9 +1711,10 @@ void ThreadScriptCheck() {
 // we're being fed a bad chain (blocks being generated much
 // too slowly or too quickly).
 //
-void PartitionCheck(bool (*initialDownloadCheck)(), CCriticalSection& cs, const CChain& chain, int64_t nPowTargetSpacing)
+void PartitionCheck(bool (*initialDownloadCheck)(), CCriticalSection& cs, const CBlockIndex *const &bestHeader,
+                    int64_t nPowTargetSpacing)
 {
-    if (initialDownloadCheck()) return;
+    if (bestHeader == NULL || initialDownloadCheck()) return;
 
     static int64_t lastAlertTime = 0;
     int64_t now = GetAdjustedTime();
@@ -1729,10 +1730,13 @@ void PartitionCheck(bool (*initialDownloadCheck)(), CCriticalSection& cs, const
     int64_t startTime = GetAdjustedTime()-SPAN_SECONDS;
 
     LOCK(cs);
-    int h = chain.Height();
-    while (h > 0 && chain[h]->GetBlockTime() >= startTime)
-        --h;
-    int nBlocks = chain.Height()-h;
+    const CBlockIndex* i = bestHeader;
+    int nBlocks = 0;
+    while (i->GetBlockTime() >= startTime) {
+        ++nBlocks;
+        i = i->pprev;
+        if (i == NULL) return; // Ran out of chain, we must not be fully sync'ed
+    }
 
     // How likely is it to find that many by chance?
     double p = boost::math::pdf(poisson, nBlocks);
diff --git a/src/main.h b/src/main.h
index abaedae207..2d39367fc6 100644
--- a/src/main.h
+++ b/src/main.h
@@ -186,7 +186,7 @@ bool SendMessages(CNode* pto, bool fSendTrickle);
 /** Run an instance of the script checking thread */
 void ThreadScriptCheck();
 /** Try to detect Partition (network isolation) attacks against us */
-void PartitionCheck(bool (*initialDownloadCheck)(), CCriticalSection& cs, const CChain& chain, int64_t nPowTargetSpacing);
+void PartitionCheck(bool (*initialDownloadCheck)(), CCriticalSection& cs, const CBlockIndex *const &bestHeader, int64_t nPowTargetSpacing);
 /** Check whether we are doing an initial block download (synchronizing from disk or network) */
 bool IsInitialBlockDownload();
 /** Format a string that describes several potential problems detected by the core */
diff --git a/src/test/alert_tests.cpp b/src/test/alert_tests.cpp
index 22cb475e02..38dcc6023c 100644
--- a/src/test/alert_tests.cpp
+++ b/src/test/alert_tests.cpp
@@ -201,7 +201,6 @@ BOOST_AUTO_TEST_CASE(PartitionAlert)
 {
     // Test PartitionCheck
     CCriticalSection csDummy;
-    CChain chainDummy;
     CBlockIndex indexDummy[100];
     CChainParams& params = Params(CBaseChainParams::MAIN);
     int64_t nPowTargetSpacing = params.GetConsensus().nPowTargetSpacing;
@@ -220,17 +219,16 @@ BOOST_AUTO_TEST_CASE(PartitionAlert)
         // Other members don't matter, the partition check code doesn't
         // use them
     }
-    chainDummy.SetTip(&indexDummy[99]);
 
     // Test 1: chain with blocks every nPowTargetSpacing seconds,
     // as normal, no worries:
-    PartitionCheck(falseFunc, csDummy, chainDummy, nPowTargetSpacing);
+    PartitionCheck(falseFunc, csDummy, &indexDummy[99], nPowTargetSpacing);
     BOOST_CHECK(strMiscWarning.empty());
 
     // Test 2: go 3.5 hours without a block, expect a warning:
     now += 3*60*60+30*60;
     SetMockTime(now);
-    PartitionCheck(falseFunc, csDummy, chainDummy, nPowTargetSpacing);
+    PartitionCheck(falseFunc, csDummy, &indexDummy[99], nPowTargetSpacing);
     BOOST_CHECK(!strMiscWarning.empty());
     BOOST_TEST_MESSAGE(std::string("Got alert text: ")+strMiscWarning);
     strMiscWarning = "";
@@ -239,7 +237,7 @@ BOOST_AUTO_TEST_CASE(PartitionAlert)
     // code:
     now += 60*10;
     SetMockTime(now);
-    PartitionCheck(falseFunc, csDummy, chainDummy, nPowTargetSpacing);
+    PartitionCheck(falseFunc, csDummy, &indexDummy[99], nPowTargetSpacing);
     BOOST_CHECK(strMiscWarning.empty());
 
     // Test 4: get 2.5 times as many blocks as expected:
@@ -248,7 +246,7 @@ BOOST_AUTO_TEST_CASE(PartitionAlert)
     int64_t quickSpacing = nPowTargetSpacing*2/5;
     for (int i = 0; i < 100; i++) // Tweak chain timestamps:
         indexDummy[i].nTime = now - (100-i)*quickSpacing;
-    PartitionCheck(falseFunc, csDummy, chainDummy, nPowTargetSpacing);
+    PartitionCheck(falseFunc, csDummy, &indexDummy[99], nPowTargetSpacing);
     BOOST_CHECK(!strMiscWarning.empty());
     BOOST_TEST_MESSAGE(std::string("Got alert text: ")+strMiscWarning);
     strMiscWarning = "";