diff --git a/src/file.cpp b/src/file.cpp
index cc0e2136c..b66b23293 100644
--- a/src/file.cpp
+++ b/src/file.cpp
@@ -1568,6 +1568,8 @@ namespace libtorrent
 #if TORRENT_HAS_FALLOCATE
 			// if fallocate failed, we have to use posix_fallocate
 			// which can be painfully slow
+			// if you get a compile error here, you might want to
+			// define TORRENT_HAS_FALLOCATE to 0.
 			ret = posix_fallocate(m_fd, 0, s);
 			if (ret != 0)
 			{