diff --git a/C/zstdmt/lz4mt_compress.c b/C/zstdmt/lz4mt_compress.c
index dcd3e6ed..cbd98b1e 100644
--- a/C/zstdmt/lz4mt_compress.c
+++ b/C/zstdmt/lz4mt_compress.c
@@ -313,6 +313,7 @@ static void *pt_compress(void *arg)
 size_t LZ4MT_compressCCtx(LZ4MT_CCtx * ctx, LZ4MT_RdWr_t * rdwr)
 {
 	int t;
+	void *retval_of_thread = 0;
 
 	if (!ctx)
 		return ERROR(compressionParameter_unsupported);
@@ -335,7 +336,7 @@ size_t LZ4MT_compressCCtx(LZ4MT_CCtx * ctx, LZ4MT_RdWr_t * rdwr)
 		void *p;
 		pthread_join(w->pthread, &p);
 		if (p)
-			return (size_t) p;
+			retval_of_thread = p;
 	}
 
 	/* clean up lists */
@@ -349,7 +350,7 @@ size_t LZ4MT_compressCCtx(LZ4MT_CCtx * ctx, LZ4MT_RdWr_t * rdwr)
 		free(wl);
 	}
 
-	return 0;
+	return (size_t)retval_of_thread;
 }
 
 /* returns current uncompressed data size */
diff --git a/C/zstdmt/lz4mt_decompress.c b/C/zstdmt/lz4mt_decompress.c
index 65c26df1..bd8997c5 100644
--- a/C/zstdmt/lz4mt_decompress.c
+++ b/C/zstdmt/lz4mt_decompress.c
@@ -485,6 +485,7 @@ size_t LZ4MT_decompressDCtx(LZ4MT_DCtx * ctx, LZ4MT_RdWr_t * rdwr)
 	int t, rv;
 	cwork_t *w = &ctx->cwork[0];
 	LZ4MT_Buffer *in = &w->in;
+	void *retval_of_thread = 0;
 
 	if (!ctx)
 		return ERROR(compressionParameter_unsupported);
@@ -544,7 +545,7 @@ size_t LZ4MT_decompressDCtx(LZ4MT_DCtx * ctx, LZ4MT_RdWr_t * rdwr)
 		void *p;
 		pthread_join(w->pthread, &p);
 		if (p)
-			return (size_t) p;
+			retval_of_thread = p;
 	}
 
  okay:
@@ -559,7 +560,7 @@ size_t LZ4MT_decompressDCtx(LZ4MT_DCtx * ctx, LZ4MT_RdWr_t * rdwr)
 		free(wl);
 	}
 
-	return 0;
+	return (size_t)retval_of_thread;
 }
 
 /* returns current uncompressed data size */
diff --git a/C/zstdmt/lz5mt_compress.c b/C/zstdmt/lz5mt_compress.c
index 6238802d..9f96e1a9 100644
--- a/C/zstdmt/lz5mt_compress.c
+++ b/C/zstdmt/lz5mt_compress.c
@@ -313,6 +313,7 @@ static void *pt_compress(void *arg)
 size_t LZ5MT_compressCCtx(LZ5MT_CCtx * ctx, LZ5MT_RdWr_t * rdwr)
 {
 	int t;
+	void *retval_of_thread = 0;
 
 	if (!ctx)
 		return ERROR(compressionParameter_unsupported);
@@ -335,7 +336,7 @@ size_t LZ5MT_compressCCtx(LZ5MT_CCtx * ctx, LZ5MT_RdWr_t * rdwr)
 		void *p;
 		pthread_join(w->pthread, &p);
 		if (p)
-			return (size_t) p;
+			retval_of_thread = p;
 	}
 
 	/* clean up lists */
@@ -349,7 +350,7 @@ size_t LZ5MT_compressCCtx(LZ5MT_CCtx * ctx, LZ5MT_RdWr_t * rdwr)
 		free(wl);
 	}
 
-	return 0;
+	return (size_t)retval_of_thread;
 }
 
 /* returns current uncompressed data size */
diff --git a/C/zstdmt/lz5mt_decompress.c b/C/zstdmt/lz5mt_decompress.c
index e2e8383f..e02c076f 100644
--- a/C/zstdmt/lz5mt_decompress.c
+++ b/C/zstdmt/lz5mt_decompress.c
@@ -485,6 +485,7 @@ size_t LZ5MT_decompressDCtx(LZ5MT_DCtx * ctx, LZ5MT_RdWr_t * rdwr)
 	int t, rv;
 	cwork_t *w = &ctx->cwork[0];
 	LZ5MT_Buffer *in = &w->in;
+	void *retval_of_thread = 0;
 
 	if (!ctx)
 		return ERROR(compressionParameter_unsupported);
@@ -544,7 +545,7 @@ size_t LZ5MT_decompressDCtx(LZ5MT_DCtx * ctx, LZ5MT_RdWr_t * rdwr)
 		void *p;
 		pthread_join(w->pthread, &p);
 		if (p)
-			return (size_t) p;
+			retval_of_thread = p;
 	}
 
  okay:
@@ -559,7 +560,7 @@ size_t LZ5MT_decompressDCtx(LZ5MT_DCtx * ctx, LZ5MT_RdWr_t * rdwr)
 		free(wl);
 	}
 
-	return 0;
+	return (size_t)retval_of_thread;
 }
 
 /* returns current uncompressed data size */
diff --git a/C/zstdmt/zstdmt_compress.c b/C/zstdmt/zstdmt_compress.c
index 9294ab56..eaaf89af 100644
--- a/C/zstdmt/zstdmt_compress.c
+++ b/C/zstdmt/zstdmt_compress.c
@@ -328,7 +328,7 @@ static void *pt_compress(void *arg)
 size_t ZSTDMT_compressCCtx(ZSTDMT_CCtx * ctx, ZSTDMT_RdWr_t * rdwr)
 {
 	int t;
-	void *rv = 0;
+	void *retval_of_thread = 0;
 
 	if (!ctx)
 		return ZSTDMT_ERROR(init_missing);
@@ -358,7 +358,7 @@ size_t ZSTDMT_compressCCtx(ZSTDMT_CCtx * ctx, ZSTDMT_RdWr_t * rdwr)
 		void *p;
 		pthread_join(w->pthread, &p);
 		if (p)
-			rv = p;
+			retval_of_thread = p;
 	}
 
 	/* clean up the free list */
@@ -373,7 +373,7 @@ size_t ZSTDMT_compressCCtx(ZSTDMT_CCtx * ctx, ZSTDMT_RdWr_t * rdwr)
 	}
 
 	/* on error, these two lists may have some entries */
-	if (rv) {
+	if (retval_of_thread) {
 		struct writelist *wl;
 		struct list_head *entry;
 
@@ -394,7 +394,7 @@ size_t ZSTDMT_compressCCtx(ZSTDMT_CCtx * ctx, ZSTDMT_RdWr_t * rdwr)
 		}
 	}
 
-	return (size_t) rv;
+	return (size_t)retval_of_thread;
 }
 
 /* returns current uncompressed data size */
diff --git a/C/zstdmt/zstdmt_decompress.c b/C/zstdmt/zstdmt_decompress.c
index 4ce0be34..1c7af2ec 100644
--- a/C/zstdmt/zstdmt_decompress.c
+++ b/C/zstdmt/zstdmt_decompress.c
@@ -697,6 +697,7 @@ size_t ZSTDMT_decompressDCtx(ZSTDMT_DCtx * ctx, ZSTDMT_RdWr_t * rdwr)
 	ZSTDMT_Buffer *in = &In;
 	cwork_t *w;
 	int t, rv, type = TYPE_UNKNOWN;
+	void *retval_of_thread = 0;
 
 	if (!ctx)
 		return ZSTDMT_ERROR(compressionParameter_unsupported);
@@ -819,7 +820,7 @@ size_t ZSTDMT_decompressDCtx(ZSTDMT_DCtx * ctx, ZSTDMT_RdWr_t * rdwr)
 		void *p;
 		pthread_join(w->pthread, &p);
 		if (p)
-			return (size_t) p;
+			retval_of_thread = p;
 	}
 
 	/* clean up pthread stuff */
@@ -838,7 +839,7 @@ size_t ZSTDMT_decompressDCtx(ZSTDMT_DCtx * ctx, ZSTDMT_RdWr_t * rdwr)
 		free(wl);
 	}
 
-	return 0;
+	return (size_t)retval_of_thread;
 }
 
 /* returns current uncompressed data size */