diff --git a/midx.c b/midx.c
index 561f65a63a..88452b0443 100644
--- a/midx.c
+++ b/midx.c
@@ -643,27 +643,26 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 	return deduplicated_entries;
 }
 
-static size_t write_midx_pack_names(struct hashfile *f,
-				    struct pack_info *info,
-				    uint32_t num_packs)
+static size_t write_midx_pack_names(struct hashfile *f, void *data)
 {
+	struct write_midx_context *ctx = data;
 	uint32_t i;
 	unsigned char padding[MIDX_CHUNK_ALIGNMENT];
 	size_t written = 0;
 
-	for (i = 0; i < num_packs; i++) {
+	for (i = 0; i < ctx->nr; i++) {
 		size_t writelen;
 
-		if (info[i].expired)
+		if (ctx->info[i].expired)
 			continue;
 
-		if (i && strcmp(info[i].pack_name, info[i - 1].pack_name) <= 0)
+		if (i && strcmp(ctx->info[i].pack_name, ctx->info[i - 1].pack_name) <= 0)
 			BUG("incorrect pack-file order: %s before %s",
-			    info[i - 1].pack_name,
-			    info[i].pack_name);
+			    ctx->info[i - 1].pack_name,
+			    ctx->info[i].pack_name);
 
-		writelen = strlen(info[i].pack_name) + 1;
-		hashwrite(f, info[i].pack_name, writelen);
+		writelen = strlen(ctx->info[i].pack_name) + 1;
+		hashwrite(f, ctx->info[i].pack_name, writelen);
 		written += writelen;
 	}
 
@@ -990,7 +989,7 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 
 		switch (chunk_ids[i]) {
 			case MIDX_CHUNKID_PACKNAMES:
-				written += write_midx_pack_names(f, ctx.info, ctx.nr);
+				written += write_midx_pack_names(f, &ctx);
 				break;
 
 			case MIDX_CHUNKID_OIDFANOUT: