diff --git a/src/test/test_bitcoin.cpp b/src/test/test_bitcoin.cpp
index f52c8ccc2..6bd228a63 100644
--- a/src/test/test_bitcoin.cpp
+++ b/src/test/test_bitcoin.cpp
@@ -69,9 +69,9 @@ TestingSetup::TestingSetup(const std::string& chainName) : BasicTestingSetup(cha
         fs::create_directories(pathTemp);
         gArgs.ForceSetArg("-datadir", pathTemp.string());
 
-        // Note that because we don't bother running a scheduler thread here,
-        // callbacks via CValidationInterface are unreliable, but that's OK,
-        // our unit tests aren't testing multiple parts of the code at once.
+        // We have to run a scheduler thread to prevent ActivateBestChain
+        // from blocking due to queue overrun.
+        threadGroup.create_thread(boost::bind(&CScheduler::serviceQueue, &scheduler));
         GetMainSignals().RegisterBackgroundSignalScheduler(scheduler);
 
         mempool.setSanityCheck(1.0);
diff --git a/src/validation.cpp b/src/validation.cpp
index 71a51c499..23ce6509d 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -40,6 +40,7 @@
 #include <validationinterface.h>
 #include <warnings.h>
 
+#include <future>
 #include <sstream>
 
 #include <boost/algorithm/string/replace.hpp>
@@ -2566,6 +2567,18 @@ bool CChainState::ActivateBestChain(CValidationState &state, const CChainParams&
     int nStopAtHeight = gArgs.GetArg("-stopatheight", DEFAULT_STOPATHEIGHT);
     do {
         boost::this_thread::interruption_point();
+
+        if (GetMainSignals().CallbacksPending() > 10) {
+            // Block until the validation queue drains. This should largely
+            // never happen in normal operation, however may happen during
+            // reindex, causing memory blowup  if we run too far ahead.
+            std::promise<void> promise;
+            CallFunctionInValidationInterfaceQueue([&promise] {
+                promise.set_value();
+            });
+            promise.get_future().wait();
+        }
+
         if (ShutdownRequested())
             break;