diff --git a/specs/lightclient/beacon-chain.md b/specs/lightclient/beacon-chain.md
index eabcb3a86..3b03ad853 100644
--- a/specs/lightclient/beacon-chain.md
+++ b/specs/lightclient/beacon-chain.md
@@ -163,7 +163,7 @@ def get_sync_committee(state: BeaconState, epoch: Epoch) -> SyncCommittee:
         bls.AggregatePKs(pubkeys[i:i + SYNC_COMMITTEE_PUBKEY_AGGREGATES_SIZE])
         for i in range(0, len(pubkeys), SYNC_COMMITTEE_PUBKEY_AGGREGATES_SIZE)
     ]
-    return SyncCommittee(pubkeys, aggregates)
+    return SyncCommittee(pubkeys=pubkeys, pubkey_aggregates=aggregates)
 ```
 
 ### Block processing
diff --git a/specs/lightclient/lightclient-fork.md b/specs/lightclient/lightclient-fork.md
index bb67fa54a..568a9793b 100644
--- a/specs/lightclient/lightclient-fork.md
+++ b/specs/lightclient/lightclient-fork.md
@@ -75,9 +75,9 @@ def upgrade_to_lightclient_patch(pre: phase0.BeaconState) -> BeaconState:
         previous_justified_checkpoint=pre.previous_justified_checkpoint,
         current_justified_checkpoint=pre.current_justified_checkpoint,
         finalized_checkpoint=pre.finalized_checkpoint,
-        # Light-client
-        current_sync_committee=SyncCommittee(),
-        next_sync_committee=SyncCommittee(),
     )
+    # Fill in sync committees
+    post.current_sync_committee = get_sync_committee(post, get_current_epoch(post))
+    post.next_sync_committee = get_sync_committee(post, get_current_epoch(post) + EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
     return post
 ```
diff --git a/tests/core/pyspec/eth2spec/test/lightclient_patch/sanity/test_blocks.py b/tests/core/pyspec/eth2spec/test/lightclient_patch/sanity/test_blocks.py
new file mode 100644
index 000000000..df8e2545c
--- /dev/null
+++ b/tests/core/pyspec/eth2spec/test/lightclient_patch/sanity/test_blocks.py
@@ -0,0 +1,102 @@
+import random
+from eth2spec.test.helpers.keys import privkeys, pubkeys
+from eth2spec.utils import bls
+from eth2spec.test.helpers.state import (
+    state_transition_and_sign_block,
+    next_epoch,
+)
+from eth2spec.test.helpers.block import (
+    build_empty_block_for_next_slot,
+)
+from eth2spec.test.context import (
+    PHASE0, PHASE1,
+    with_all_phases_except,
+    spec_state_test,
+)
+
+
+def compute_light_client_signature(spec, state, slot, privkey):
+    domain = spec.get_domain(state, spec.DOMAIN_SYNC_COMMITTEE, spec.compute_epoch_at_slot(slot))
+    if slot == state.slot:
+        block_root = build_empty_block_for_next_slot(spec, state).parent_root
+    else:
+        block_root = spec.get_block_root_at_slot(state, slot)
+    signing_root = spec.compute_signing_root(block_root, domain)
+    return bls.Sign(privkey, signing_root)
+
+
+def compute_aggregate_light_client_signature(spec, state, slot, participants):
+    if len(participants) == 0:
+        return spec.G2_POINT_AT_INFINITY
+
+    signatures = []
+    for validator_index in participants:
+        privkey = privkeys[validator_index]
+        signatures.append(
+            compute_light_client_signature(
+                spec,
+                state,
+                slot,
+                privkey,
+            )
+        )
+    return bls.Aggregate(signatures)
+
+
+def run_light_client_sanity_test(spec, state, fraction_full=1.0):
+    committee = spec.get_sync_committee_indices(state, spec.get_current_epoch(state))
+    participants = random.sample(committee, int(len(committee) * fraction_full))
+
+    yield 'pre', state
+
+    block = build_empty_block_for_next_slot(spec, state)
+    block.body.sync_committee_bits = [index in participants for index in committee]
+    block.body.sync_committee_signature = compute_aggregate_light_client_signature(
+        spec,
+        state,
+        block.slot - 1,
+        participants,
+    )
+    signed_block = state_transition_and_sign_block(spec, state, block)
+
+    yield 'blocks', [signed_block]
+    yield 'post', state
+
+
+@with_all_phases_except([PHASE0, PHASE1])
+@spec_state_test
+def test_full_light_client_committee(spec, state):
+    next_epoch(spec, state)
+    yield from run_light_client_sanity_test(spec, state, fraction_full=1.0)
+
+
+@with_all_phases_except([PHASE0, PHASE1])
+@spec_state_test
+def test_half_light_client_committee(spec, state):
+    next_epoch(spec, state)
+    yield from run_light_client_sanity_test(spec, state, fraction_full=0.5)
+
+
+@with_all_phases_except([PHASE0, PHASE1])
+@spec_state_test
+def test_empty_light_client_committee(spec, state):
+    next_epoch(spec, state)
+    yield from run_light_client_sanity_test(spec, state, fraction_full=0.0)
+
+
+@with_all_phases_except([PHASE0, PHASE1])
+@spec_state_test
+def test_full_light_client_committee_genesis(spec, state):
+    yield from run_light_client_sanity_test(spec, state, fraction_full=1.0)
+
+
+@with_all_phases_except([PHASE0, PHASE1])
+@spec_state_test
+def test_half_light_client_committee_genesis(spec, state):
+    yield from run_light_client_sanity_test(spec, state, fraction_full=0.5)
+
+
+@with_all_phases_except([PHASE0, PHASE1])
+@spec_state_test
+def test_empty_light_client_committee_genesis(spec, state):
+    yield from run_light_client_sanity_test(spec, state, fraction_full=0.0)