diff --git a/mkvparser.cpp b/mkvparser.cpp
index e48343f..01c932e 100644
--- a/mkvparser.cpp
+++ b/mkvparser.cpp
@@ -7962,7 +7962,6 @@ Cluster::GetEntry(
         }
 
         const long long tc_ = pBlock->GetTimeCode(this);
-        assert(tc_ >= 0);
 
         if (tc_ < tc)
         {
@@ -8081,7 +8080,7 @@ SimpleBlock::SimpleBlock(
 
 long SimpleBlock::Parse()
 {
-    return m_block.Parse(m_pCluster->m_pSegment->m_pReader);
+    return m_block.Parse(m_pCluster);
 }
 
 
@@ -8116,7 +8115,7 @@ BlockGroup::BlockGroup(
 
 long BlockGroup::Parse()
 {
-    const long status = m_block.Parse(m_pCluster->m_pSegment->m_pReader);
+    const long status = m_block.Parse(m_pCluster);
 
     if (status)
         return status;
@@ -8186,9 +8185,14 @@ Block::~Block()
 }
 
 
-long Block::Parse(IMkvReader* pReader)
+long Block::Parse(const Cluster* pCluster)
 {
-    assert(pReader);
+    if (pCluster == NULL)
+        return -1;
+
+    if (pCluster->m_pSegment == NULL)
+        return -1;
+
     assert(m_start >= 0);
     assert(m_size >= 0);
     assert(m_track <= 0);
@@ -8200,6 +8204,8 @@ long Block::Parse(IMkvReader* pReader)
 
     long len;
 
+    IMkvReader* const pReader = pCluster->m_pSegment->m_pReader;
+
     m_track = ReadUInt(pReader, pos, len);
 
     if (m_track <= 0)
@@ -8528,7 +8534,6 @@ long long Block::GetTimeCode(const Cluster* pCluster) const
     assert(tc0 >= 0);
 
     const long long tc = tc0 + m_timecode;
-    assert(tc >= 0);
 
     return tc;  //unscaled timecode units
 }
diff --git a/mkvparser.hpp b/mkvparser.hpp
index 7e4abec..76f79bb 100644
--- a/mkvparser.hpp
+++ b/mkvparser.hpp
@@ -86,7 +86,7 @@ public:
     Block(long long start, long long size);
     ~Block();
 
-    long Parse(IMkvReader*);
+    long Parse(const Cluster*);
 
     long long GetTrackNumber() const;
     long long GetTimeCode(const Cluster*) const;  //absolute, but not scaled