diff --git a/lib/zstd.h b/lib/zstd.h
index 291c6df25..a2a756dfc 100644
--- a/lib/zstd.h
+++ b/lib/zstd.h
@@ -955,7 +955,9 @@ typedef enum {
                               * Special: value 0 means "do not change strategy". */
 
     /* frame parameters */
-    ZSTD_p_contentSizeFlag=200, /* Content size is written into frame header _whenever known_ (default:1) */
+    ZSTD_p_contentSizeFlag=200, /* Content size is written into frame header _whenever known_ (default:1)
+                              * note that content size must be known at the beginning,
+                              * it is sent using ZSTD_CCtx_setPledgedSrcSize() */
     ZSTD_p_checksumFlag,     /* A 32-bits checksum of content is written at end of frame (default:0) */
     ZSTD_p_dictIDFlag,       /* When applicable, dictID of dictionary is provided in frame header (default:1) */