diff --git a/specs/core/0_fork-choice.md b/specs/core/0_fork-choice.md
index 3ab041837..71920d2bc 100644
--- a/specs/core/0_fork-choice.md
+++ b/specs/core/0_fork-choice.md
@@ -149,14 +149,17 @@ def on_tick(store: Store, time: int) -> None:
 ```python
 def on_block(store: Store, block: BeaconBlock) -> None:
     # Make a copy of the state to avoid mutability issues
-    parent_block = store.blocks[block.parent_root]
+    assert block.parent_root in store.block_states
     pre_state = store.block_states[block.parent_root].copy()
     # Blocks cannot be in the future. If they are, their consideration must be delayed until the are in the past.
     assert store.time >= pre_state.genesis_time + block.slot * SECONDS_PER_SLOT
     # Add new block to the store
     store.blocks[signing_root(block)] = block
     # Check block is a descendant of the finalized block
-    assert get_ancestor(store, signing_root(block), store.blocks[store.finalized_checkpoint.root].slot) == store.finalized_checkpoint.root
+    assert (
+        get_ancestor(store, signing_root(block), store.blocks[store.finalized_checkpoint.root].slot) ==
+        store.finalized_checkpoint.root
+    )
     # Check that block is later than the finalized epoch slot
     assert block.slot > get_epoch_start_slot(store.finalized_checkpoint.epoch)
     # Check the block is valid and compute the post-state
@@ -184,13 +187,14 @@ def on_attestation(store: Store, attestation: Attestation) -> None:
     # Cannot calculate the current shuffling if have not seen the target
     assert target.root in store.blocks
 
-    # Attestations cannot be from future epochs. If they are, their consideration must be delayed until the are in the past.
+    # Attestations cannot be from future epochs. If they are, delay consideration until the epoch arrivesr
     base_state = store.block_states[target.root].copy()
     assert store.time >= base_state.genesis_time + get_epoch_start_slot(target.epoch) * SECONDS_PER_SLOT
 
     # Store target checkpoint state if not yet seen
     if target not in store.checkpoint_states:
-        store.checkpoint_states[target] = process_slots(base_state, get_epoch_start_slot(target.epoch))
+        process_slots(base_state, get_epoch_start_slot(target.epoch))
+        store.checkpoint_states[target] = base_state
     target_state = store.checkpoint_states[target]
 
     # Attestations can only affect the fork choice of subsequent slots.
diff --git a/test_libs/pyspec/eth2spec/test/fork_choice/test_on_attestation.py b/test_libs/pyspec/eth2spec/test/fork_choice/test_on_attestation.py
index fd42a6b29..cf0e7c9cb 100644
--- a/test_libs/pyspec/eth2spec/test/fork_choice/test_on_attestation.py
+++ b/test_libs/pyspec/eth2spec/test/fork_choice/test_on_attestation.py
@@ -11,7 +11,7 @@ def run_on_attestation(spec, state, store, attestation, valid=True):
     if not valid:
         try:
             spec.on_attestation(store, attestation)
-        except:
+        except AssertionError:
             return
         else:
             assert False
@@ -131,4 +131,4 @@ def test_on_attestation_invalid_attestation(spec, state):
     attestation = get_valid_attestation(spec, state, slot=block.slot)
     # make attestation invalid
     attestation.custody_bitfield = b'\xf0' + attestation.custody_bitfield[1:]
-    run_on_attestation(spec, state, store, attestation, False)
\ No newline at end of file
+    run_on_attestation(spec, state, store, attestation, False)
diff --git a/test_libs/pyspec/eth2spec/test/fork_choice/test_on_block.py b/test_libs/pyspec/eth2spec/test/fork_choice/test_on_block.py
index ef25317b6..b18752f8c 100644
--- a/test_libs/pyspec/eth2spec/test/fork_choice/test_on_block.py
+++ b/test_libs/pyspec/eth2spec/test/fork_choice/test_on_block.py
@@ -9,7 +9,7 @@ def run_on_block(spec, state, store, block, valid=True):
     if not valid:
         try:
             spec.on_block(store, block)
-        except:
+        except AssertionError:
             return
         else:
             assert False
@@ -88,8 +88,11 @@ def test_on_block_before_finalized(spec, state):
     time = 100
     spec.on_tick(store, time)
 
-    store.finalized_checkpoint = spec.Checkpoint(epoch=store.finalized_checkpoint.epoch + 2, root=store.finalized_checkpoint.root)
+    store.finalized_checkpoint = spec.Checkpoint(
+        epoch=store.finalized_checkpoint.epoch + 2,
+        root=store.finalized_checkpoint.root
+    )
 
     # Fail receiving block of `GENESIS_SLOT + 1` slot
     block = build_empty_block_for_next_slot(spec, state)
-    run_on_block(spec, state, store, block, False)
\ No newline at end of file
+    run_on_block(spec, state, store, block, False)
diff --git a/test_libs/pyspec/eth2spec/test/test_fork_choice.py b/test_libs/pyspec/eth2spec/test/test_fork_choice.py
deleted file mode 100644
index 8b1378917..000000000
--- a/test_libs/pyspec/eth2spec/test/test_fork_choice.py
+++ /dev/null
@@ -1 +0,0 @@
-