diff --git a/Source/CLI/Main.cpp b/Source/CLI/Main.cpp
index 08439409..ed05bafb 100644
--- a/Source/CLI/Main.cpp
+++ b/Source/CLI/Main.cpp
@@ -465,6 +465,15 @@ int ParseFile_Uncompressed(parse_info& ParseInfo, size_t Files_Pos)
 
         Output.Streams.push_back(Stream);
 
+        // Pre-set frame_size so ffmpeg's enc_open() does not set it to nb_samples,
+        // which the FLAC encoder would reject as an invalid block size
+        // if the audio stream has fewer samples than FLAC's minimum block size (16).
+        if (ParseInfo.InputInfo.SampleCount > 0 && ParseInfo.InputInfo.SampleCount < 16)
+        {
+            if (Global.OutputOptions.find("frame_size") == Global.OutputOptions.end())
+                Global.OutputOptions["frame_size"] = "16";
+        }
+
         if (Global.Actions[Action_Coherency])
         {
             // Duration
diff --git a/Source/Lib/Compressed/Matroska/Matroska.cpp b/Source/Lib/Compressed/Matroska/Matroska.cpp
index e3772f9b..117acaa0 100644
--- a/Source/Lib/Compressed/Matroska/Matroska.cpp
+++ b/Source/Lib/Compressed/Matroska/Matroska.cpp
@@ -197,10 +197,15 @@ ELEMENT_VOID(       6, Segment_Attachments_AttachedFile_FileData_RawCookedTrack_
 ELEMENT_END()
 
 ELEMENT_BEGIN(Segment_Cluster)
+ELEMENT_CASE(      20, Segment_Cluster_BlockGroup)
 ELEMENT_VOID(      23, Segment_Cluster_SimpleBlock)
 ELEMENT_VOID(      67, Segment_Cluster_Timestamp)
 ELEMENT_END()
 
+ELEMENT_BEGIN(Segment_Cluster_BlockGroup)
+ELEMENT_VOID(      21, Segment_Cluster_BlockGroup_Block)
+ELEMENT_END()
+
 ELEMENT_BEGIN(Segment_Tracks)
 ELEMENT_CASE(      2E, Segment_Tracks_TrackEntry)
 ELEMENT_END()
@@ -930,8 +935,26 @@ void matroska::Segment_Cluster()
     }
 }
 
+//---------------------------------------------------------------------------
+void matroska::Segment_Cluster_BlockGroup()
+{
+    IsList = true;
+}
+
+//---------------------------------------------------------------------------
+void matroska::Segment_Cluster_BlockGroup_Block()
+{
+    ProcessBlock();
+}
+
 //---------------------------------------------------------------------------
 void matroska::Segment_Cluster_SimpleBlock()
+{
+    ProcessBlock();
+}
+
+//---------------------------------------------------------------------------
+void matroska::ProcessBlock()
 {
     if (Levels[Level].Offset_End - Buffer_Offset > 4)
     {
diff --git a/Source/Lib/Compressed/Matroska/Matroska.h b/Source/Lib/Compressed/Matroska/Matroska.h
index c112b765..a16f5645 100644
--- a/Source/Lib/Compressed/Matroska/Matroska.h
+++ b/Source/Lib/Compressed/Matroska/Matroska.h
@@ -146,6 +146,8 @@ class matroska : public input_base
     MATROSKA_ELEMENT(Segment_Attachments_AttachedFile_FileData_RawCookedTrack_LibraryName);
     MATROSKA_ELEMENT(Segment_Attachments_AttachedFile_FileData_RawCookedTrack_LibraryVersion);
     MATROSKA_ELEMENT(Segment_Cluster);
+    MATROSKA_ELEMENT(Segment_Cluster_BlockGroup);
+    MATROSKA_ELEMENT(Segment_Cluster_BlockGroup_Block);
     MATROSKA_ELEMENT(Segment_Cluster_SimpleBlock);
     MATROSKA_ELEMENT(Segment_Cluster_Timestamp);
     MATROSKA_ELEMENT(Segment_Tracks);
@@ -192,6 +194,7 @@ class matroska : public input_base
     int16_t                     Block_Timestamp;
 
     //Utils
+    void                        ProcessBlock();
     void                        Uncompress(buffer& Buffer);
     void                        Segment_Attachments_AttachedFile_FileData_RawCookedxxx_yyy(reversibility::element Element, type Type);
     void                        StoreFromCurrentToEndOfElement(buffer& Output);