diff --git a/plot_transmission_timeline.py b/plot_transmission_timeline.py
index 68bdc1c..a6fc112 100755
--- a/plot_transmission_timeline.py
+++ b/plot_transmission_timeline.py
@@ -151,7 +151,7 @@ if __name__ == "__main__":
     range_sum_interval = "{}s".format(args.interval)
 
     # create timedelta range with maximum timedelta
-    time_range = pd.timedelta_range(range_start_time, transmission_df.index.max(), freq=range_sum_interval)
+    time_range = pd.timedelta_range(pd.to_timedelta(range_start_time), transmission_df.index.max(), freq=range_sum_interval)
 
     # create bins by pd.cut, aggregate sum
     transmission_df = transmission_df.groupby(pd.cut(transmission_df.index, bins=time_range, labels=time_range[:-1]))["goodput"].sum().reset_index()