diff --git a/src/index/base.cpp b/src/index/base.cpp
--- a/src/index/base.cpp
+++ b/src/index/base.cpp
@@ -96,6 +96,9 @@
                     prune_violation = false;
                     break;
                 }
+                // block->pprev must exist at this point, since block_to_test is
+                // part of the chain and thus must be encountered when going
+                // backwards from the tip
                 assert(block->pprev);
                 block = block->pprev;
             }
diff --git a/test/functional/feature_blockfilterindex_prune.py b/test/functional/feature_blockfilterindex_prune.py
--- a/test/functional/feature_blockfilterindex_prune.py
+++ b/test/functional/feature_blockfilterindex_prune.py
@@ -39,6 +39,9 @@
 
         self.log.info("prune some blocks")
         pruneheight = node.pruneblockchain(400)
+        # The prune heights used here and below are magic numbers that are
+        # determined by the thresholds at which block files wrap, so they depend
+        # on disk serialization and default block file size.
         # The difference in number of blocks per block file between Bitcoin ABC
         # and Bitcoin Core is caused by additional witness data in coinbase
         # transactions for core.