diff --git a/src/chain.cpp b/src/chain.cpp
index 32f6480f8..77e924e70 100644
--- a/src/chain.cpp
+++ b/src/chain.cpp
@@ -93,6 +93,7 @@ CBlockIndex* CBlockIndex::GetAncestor(int height)
             pindexWalk = pindexWalk->pskip;
             heightWalk = heightSkip;
         } else {
+            assert(pindexWalk->pprev);
             pindexWalk = pindexWalk->pprev;
             heightWalk--;
         }