diff --git a/lib/compress/zstdmt_compress.c b/lib/compress/zstdmt_compress.c
index 9fea4969..7b37c5b3 100644
--- a/lib/compress/zstdmt_compress.c
+++ b/lib/compress/zstdmt_compress.c
@@ -142,6 +142,10 @@ static size_t ZSTDMT_sizeof_bufferPool(ZSTDMT_bufferPool* bufPool)
     return poolSize + totalBufferSize;
 }
 
+/* ZSTDMT_setBufferSize() :
+ * all future buffers provided by this buffer pool will have _at least_ this size
+ * note : it's better for all buffers to have same size,
+ * as they become freely interchangeable, reducing malloc/free usages and memory fragmentation */
 static void ZSTDMT_setBufferSize(ZSTDMT_bufferPool* const bufPool, size_t const bSize)
 {
     ZSTD_pthread_mutex_lock(&bufPool->poolMutex);
@@ -992,12 +996,11 @@ size_t ZSTDMT_initCStream(ZSTDMT_CCtx* mtctx, int compressionLevel) {
 
 
 /* ZSTDMT_writeLastEmptyBlock()
- * Write a single empty block with an end-of-frame
- * to finish a frame.
- * Completed synchronously.
- * @return : 0, or an error code (can fail due to memory allocation)
+ * Write a single empty block with an end-of-frame to finish a frame.
+ * Job must be created from streaming variant.
+ * This function is always successfull if expected conditions are fulfilled.
  */
-static size_t ZSTDMT_writeLastEmptyBlock(ZSTDMT_jobDescription* job)
+static void ZSTDMT_writeLastEmptyBlock(ZSTDMT_jobDescription* job)
 {
     assert(job->lastChunk == 1);
     assert(job->srcSize == 0);      /* last chunk is empty -> will be simplified into a last empty block */
@@ -1006,15 +1009,14 @@ static size_t ZSTDMT_writeLastEmptyBlock(ZSTDMT_jobDescription* job)
      * It summons a dstBuffer itself, compresses into it, then releases srcBuffer, and gives result to mtctx.
      * When done, srcBuffer is empty, while dstBuffer is filled, and will be released by mtctx.
      * This shortcut will simply switch srcBuffer for dstBuffer, providing same outcome as a normal job */
-    assert(job->dstBuff.start == NULL);  /* invoked from streaming variant only (otherwise, dstBuff might be user's output) */
-    assert(job->srcBuff.start != NULL);      /* invoked from streaming variant only (otherwise, srcBuff might be user's input) */
-    assert(job->srcBuff.size >= ZSTD_blockHeaderSize);
+    assert(job->dstBuff.start == NULL);   /* invoked from streaming variant only (otherwise, dstBuff might be user's output) */
+    assert(job->srcBuff.start != NULL);   /* invoked from streaming variant only (otherwise, srcBuff might be user's input) */
+    assert(job->srcBuff.size >= ZSTD_blockHeaderSize);   /* no buffer should ever be that small */
     job->dstBuff = job->srcBuff;
     job->srcBuff = g_nullBuffer;
     job->cSize = ZSTD_writeLastEmptyBlock(job->dstBuff.start, job->dstBuff.size);
     assert(!ZSTD_isError(job->cSize));
     assert(job->consumed == 0);
-    return 0;
 }
 
 static size_t ZSTDMT_createCompressionJob(ZSTDMT_CCtx* mtctx, size_t srcSize, ZSTD_EndDirective endOp)
@@ -1031,6 +1033,7 @@ static size_t ZSTDMT_createCompressionJob(ZSTDMT_CCtx* mtctx, size_t srcSize, ZS
     if (!mtctx->jobReady) {
         DEBUGLOG(5, "ZSTDMT_createCompressionJob: preparing job %u to compress %u bytes with %u preload ",
                     mtctx->nextJobID, (U32)srcSize, (U32)mtctx->prefixSize);
+        assert(mtctx->jobs[jobID].srcBuff.start == NULL);   /* no buffer left : supposed already released */
         mtctx->jobs[jobID].srcBuff = mtctx->inBuff.buffer;
         mtctx->jobs[jobID].prefixStart = mtctx->inBuff.buffer.start;
         mtctx->jobs[jobID].prefixSize = mtctx->prefixSize;
@@ -1085,7 +1088,7 @@ static size_t ZSTDMT_createCompressionJob(ZSTDMT_CCtx* mtctx, size_t srcSize, ZS
         if ( (srcSize == 0)
           && (mtctx->nextJobID>0)/*single chunk must also write frame header*/ ) {
             assert(endOp == ZSTD_e_end);  /* only possible case : need to end the frame with an empty last block */
-            CHECK_F( ZSTDMT_writeLastEmptyBlock(mtctx->jobs + jobID) );
+            ZSTDMT_writeLastEmptyBlock(mtctx->jobs + jobID);
             mtctx->nextJobID++;
             return 0;
         }
@@ -1162,10 +1165,11 @@ static size_t ZSTDMT_flushProduced(ZSTDMT_CCtx* mtctx, ZSTD_outBuffer* output, u
             output->pos += toWrite;
             job.dstFlushed += toWrite;
 
-            if ( (job.consumed == job.srcSize)
-              && (job.dstFlushed == job.cSize) ) {   /* output buffer fully flushed => move to next one */
+            if ( (job.consumed == job.srcSize)      /* job completed */
+              && (job.dstFlushed == job.cSize) ) {  /* output buffer fully flushed => free this job position */
                 DEBUGLOG(5, "Job %u completed (%u bytes), moving to next one",
                         mtctx->doneJobID, (U32)job.dstFlushed);
+                assert(job.srcBuff.start == NULL);  /* srcBuff supposed already released */
                 ZSTDMT_releaseBuffer(mtctx->bufPool, job.dstBuff);
                 mtctx->jobs[wJobID].dstBuff = g_nullBuffer;
                 mtctx->consumed += job.srcSize;