diff -up dovecot-2.3.8/src/lib-storage/index/index-mail-binary.c.blockcount dovecot-2.3.8/src/lib-storage/index/index-mail-binary.c

--- dovecot-2.3.8/src/lib-storage/index/index-mail-binary.c.blockcount	2020-12-02 11:34:10.229027593 +0100

+++ dovecot-2.3.8/src/lib-storage/index/index-mail-binary.c	2020-12-02 11:36:47.328933276 +0100

@@ -339,13 +339,14 @@ blocks_count_lines(struct binary_ctx *ct

 		i_stream_skip(full_input, skip);

 		cur_block_offset += skip;

-		if (cur_block->input->eof) {

+		if (i_stream_read_eof(cur_block->input)) {

 			/* go to the next block */

-			if (++block_idx == block_count) {

+			if (block_idx+1 == block_count) {

 				i_assert(i_stream_read_eof(full_input));

 				ret = -1;

 				break;

 			}

+			block_idx++;

 			cur_block++;

 			cur_block_offset = 0;

 		}