diff --git a/include/unarchive.h b/include/unarchive.h
index 11d8c77..ba6d323 100644
--- a/include/unarchive.h
+++ b/include/unarchive.h
@@ -194,6 +194,8 @@
 /* A bit of bunzip2 internals are exposed for compressed help support: */
 typedef struct bunzip_data bunzip_data;
 int start_bunzip(bunzip_data **bdp, int in_fd, const void *inbuf, int len) FAST_FUNC;
+/* NB: read_bunzip returns < 0 on error, or the number of *unfilled* bytes
+ * in outbuf. IOW: on EOF returns len ("all bytes are not filled"), not 0: */
 int read_bunzip(bunzip_data *bd, char *outbuf, int len) FAST_FUNC;
 void dealloc_bunzip(bunzip_data *bd) FAST_FUNC;