diff --git a/src/spikeinterface/metrics/quality/misc_metrics.py b/src/spikeinterface/metrics/quality/misc_metrics.py
index 22e5a790cc..f985ea6e23 100644
--- a/src/spikeinterface/metrics/quality/misc_metrics.py
+++ b/src/spikeinterface/metrics/quality/misc_metrics.py
@@ -321,17 +321,16 @@ def compute_refrac_period_violations(
 
     res = namedtuple("rp_violations", ["rp_contamination", "rp_violations"])
 
+    if unit_ids is None:
+        unit_ids = sorting_analyzer.unit_ids
+
     if not HAVE_NUMBA:
         warnings.warn("Error: numba is not installed.")
         warnings.warn("compute_refrac_period_violations cannot run without numba.")
-        return None
+        return {unit_id: np.nan for unit_id in unit_ids}
 
     sorting = sorting_analyzer.sorting
     fs = sorting_analyzer.sampling_frequency
-
-    if unit_ids is None:
-        unit_ids = sorting_analyzer.unit_ids
-
     num_spikes = sorting.count_num_spikes_per_unit(unit_ids=unit_ids)
 
     t_c = int(round(censored_period_ms * fs * 1e-3))