diff --git a/piker/data/_timeseries.py b/piker/data/_timeseries.py
index 7055854b..cc84f763 100644
--- a/piker/data/_timeseries.py
+++ b/piker/data/_timeseries.py
@@ -209,8 +209,13 @@ def detect_null_time_gap(
     NOTE: for now presumes only ONE gap XD
 
     '''
-    zero_pred: np.ndarray = shm.array['time'] == 0
-    zero_t: np.ndarray = shm.array[zero_pred]
+    # ensure we read buffer state only once so that ShmArray rt
+    # circular-buffer updates don't cause a indexing/size mismatch.
+    array: np.ndarray = shm.array
+
+    zero_pred: np.ndarray = array['time'] == 0
+    zero_t: np.ndarray = array[zero_pred]
+
     if zero_t.size:
         istart, iend = zero_t['index'][[0, -1]]
         start, end = shm._array['time'][