diff --git a/specs/phase0/fork-choice.md b/specs/phase0/fork-choice.md
index 6a5437115..78d3eca02 100644
--- a/specs/phase0/fork-choice.md
+++ b/specs/phase0/fork-choice.md
@@ -623,6 +623,9 @@ def update_latest_messages(store: Store, attesting_indices: Sequence[ValidatorIn
 
 ```python
 def on_tick(store: Store, time: uint64) -> None:
+    # Precondition
+    assert time >= store.time
+
     # If the ``store.time`` falls behind, while loop catches up slot by slot
     # to ensure that every previous slot is processed with ``on_tick_per_slot``
     tick_slot = (time - store.genesis_time) // SECONDS_PER_SLOT
diff --git a/tests/core/pyspec/eth2spec/test/helpers/fork_choice.py b/tests/core/pyspec/eth2spec/test/helpers/fork_choice.py
index ef80a3ab6..bc40c1f6d 100644
--- a/tests/core/pyspec/eth2spec/test/helpers/fork_choice.py
+++ b/tests/core/pyspec/eth2spec/test/helpers/fork_choice.py
@@ -144,9 +144,10 @@ def get_blobs_file_name(blobs=None, blobs_root=None):
 
 
 def on_tick_and_append_step(spec, store, time, test_steps):
-    spec.on_tick(store, time)
-    test_steps.append({'tick': int(time)})
-    output_store_checks(spec, store, test_steps)
+    if store.time < time:
+        spec.on_tick(store, time)
+        test_steps.append({'tick': int(time)})
+        output_store_checks(spec, store, test_steps)
 
 
 def run_on_block(spec, store, signed_block, valid=True):