diff --git a/neurokit2/ppg/ppg_intervalrelated.py b/neurokit2/ppg/ppg_intervalrelated.py
index 7f91483140..fdee4fd9d8 100644
--- a/neurokit2/ppg/ppg_intervalrelated.py
+++ b/neurokit2/ppg/ppg_intervalrelated.py
@@ -86,6 +86,10 @@ def ppg_intervalrelated(data, sampling_rate=1000):
             )
 
         ppg_intervals = pd.DataFrame.from_dict(intervals, orient="index")
+    else:
+        raise ValueError(
+            "Input should be a dataframe or a dict of dataframes (epochs)."
+        )
 
     return ppg_intervals