diff --git a/src/interfaces/chain.cpp b/src/interfaces/chain.cpp
--- a/src/interfaces/chain.cpp
+++ b/src/interfaces/chain.cpp
@@ -282,6 +282,8 @@
             const CBlockIndex *block2 = LookupBlockIndex(block_hash2);
             const CBlockIndex *ancestor =
                 block1 && block2 ? LastCommonAncestor(block1, block2) : nullptr;
+            // Using & instead of && below to avoid short circuiting and leaving
+            // output uninitialized.
             return FillBlock(ancestor, ancestor_out, lock) &
                    FillBlock(block1, block1_out, lock) &
                    FillBlock(block2, block2_out, lock);
diff --git a/src/test/interfaces_tests.cpp b/src/test/interfaces_tests.cpp
--- a/src/test/interfaces_tests.cpp
+++ b/src/test/interfaces_tests.cpp
@@ -131,6 +131,16 @@
     BOOST_CHECK_EQUAL(orig_height, orig_tip->nHeight);
     BOOST_CHECK_EQUAL(fork_height, orig_tip->nHeight - 10);
     BOOST_CHECK_EQUAL(fork_hash, active[fork_height]->GetBlockHash());
+
+    BlockHash active_hash, orig_hash;
+    BOOST_CHECK(!chain->findCommonAncestor(active.Tip()->GetBlockHash(),
+                                           BlockHash(), {},
+                                           FoundBlock().hash(active_hash), {}));
+    BOOST_CHECK(!chain->findCommonAncestor(BlockHash(),
+                                           orig_tip->GetBlockHash(), {}, {},
+                                           FoundBlock().hash(orig_hash)));
+    BOOST_CHECK_EQUAL(active_hash, active.Tip()->GetBlockHash());
+    BOOST_CHECK_EQUAL(orig_hash, orig_tip->GetBlockHash());
 }
 
 BOOST_AUTO_TEST_CASE(hasBlocks) {