diff --git a/src/timeseries.cpp b/src/timeseries.cpp
index 5dd0d60..6891fbe 100644
--- a/src/timeseries.cpp
+++ b/src/timeseries.cpp
@@ -27,11 +27,12 @@ void timeseries_prune(timeseries_t *ts, uint64_t duration)
 
 	uint64_t new_tstart = ts->tend - duration;
 
-	while(iter != ts->data.end() && iter->second < new_tstart) {
+	while(iter != ts->data.end() && iter->first < new_tstart) {
 		iter = ts->data.erase(iter);
 	}
 
 	if(iter != ts->data.end()) {
+		// iter now points to the first entry in the list
 		ts->tstart = iter->first;
 	} else {
 		ts->tstart = ts->tend;