diff --git a/srtcore/buffer_rcv.cpp b/srtcore/buffer_rcv.cpp
index a15e2f63d..e10e7e319 100644
--- a/srtcore/buffer_rcv.cpp
+++ b/srtcore/buffer_rcv.cpp
@@ -547,6 +547,11 @@ int CRcvBuffer::getTimespan_ms() const
         return 0;
 
     const int lastpos = incPos(m_iStartPos, m_iMaxPosInc - 1);
+    // Should not happen if TSBPD is enabled (reading out of order is not allowed).
+    SRT_ASSERT(m_entries[lastpos].pUnit != NULL);
+    if (m_entries[lastpos].pUnit == NULL)
+        return 0;
+
     int startpos = m_iStartPos;
 
     while (m_entries[startpos].pUnit == NULL)
@@ -560,11 +565,6 @@ int CRcvBuffer::getTimespan_ms() const
     if (m_entries[startpos].pUnit == NULL)
         return 0;
 
-    // Should not happen
-    SRT_ASSERT(m_entries[lastpos].pUnit != NULL);
-    if (m_entries[lastpos].pUnit == NULL)
-        return 0;
-
     const steady_clock::time_point startstamp =
         getPktTsbPdTime(m_entries[startpos].pUnit->m_Packet.getMsgTimeStamp());
     const steady_clock::time_point endstamp = getPktTsbPdTime(m_entries[lastpos].pUnit->m_Packet.getMsgTimeStamp());