diff --git a/include/sys/zio_compress.h b/include/sys/zio_compress.h
index 4c0d920fc2..6e96ed6cb3 100644
--- a/include/sys/zio_compress.h
+++ b/include/sys/zio_compress.h
@@ -182,6 +182,8 @@ extern int lz4_decompress_zfs(void *src, void *dst, size_t s_len, size_t d_len,
     int level);
 extern size_t slack_compress(void *src, void *dst, size_t s_len, size_t d_len,
     int level);
+extern int slack_decompress(void *src, void *dst, size_t s_len, size_t d_len,
+    int level);
 
 /*
  * Compress and decompress data if necessary.
diff --git a/module/zfs/slack.c b/module/zfs/slack.c
index 6c9a041806..cbf797dd9c 100644
--- a/module/zfs/slack.c
+++ b/module/zfs/slack.c
@@ -54,3 +54,11 @@ slack_compress(void *src, void *dst, size_t s_len, size_t d_len, int level)
 	memcpy(dst, src, c_len);
 	return (c_len);
 }
+
+int
+slack_decompress(void *src, void *dst, size_t s_len, size_t d_len, int level)
+{
+	ASSERT3U(d_len, >=, s_len);
+	memcpy(dst, src, s_len);
+	return (0);
+}
diff --git a/module/zfs/zio_compress.c b/module/zfs/zio_compress.c
index 7c8188f7d7..cafcc8879e 100644
--- a/module/zfs/zio_compress.c
+++ b/module/zfs/zio_compress.c
@@ -68,7 +68,7 @@ zio_compress_info_t zio_compress_table[ZIO_COMPRESS_FUNCTIONS] = {
 	{"lz4",		0,	lz4_compress_zfs, lz4_decompress_zfs, NULL},
 	{"zstd",	ZIO_ZSTD_LEVEL_DEFAULT,	zfs_zstd_compress,
 	    zfs_zstd_decompress, zfs_zstd_decompress_level},
-	{"slack",	0,	slack_compress,	NULL, NULL },
+	{"slack",	0,	slack_compress,	slack_decompress, NULL },
 };
 
 uint8_t