diff --git a/src/core/tsi/transport_security_interface.h b/src/core/tsi/transport_security_interface.h
index 936b0c25b0a8be66f32cd9131417443b8dc610a6..e27e6b9fc92950aecfe66add7b4aaafe08f20588 100644
--- a/src/core/tsi/transport_security_interface.h
+++ b/src/core/tsi/transport_security_interface.h
@@ -158,6 +158,8 @@ tsi_result tsi_frame_protector_protect_flush(
      value is expected to be at most max_protected_frame_size minus overhead
      which means that max_protected_frame_size is a safe bet. The output value
      is the number of bytes actually written.
+     If *unprotected_bytes_size is unchanged, there may be more data remaining
+     to unprotect, and the caller should call this function again.
 
    - This method returns TSI_OK in case of success. Success includes cases where
      there is not enough data to output a frame in which case