diff --git a/src/trees-istream.c b/src/trees-istream.c
index e94be7d2bbb54906be3dd737abec78561841ab44..343091db9d71f231e7d71c765e93aca0b4b363c1 100644
--- a/src/trees-istream.c
+++ b/src/trees-istream.c
@@ -138,7 +138,7 @@ trees_istream_read_detect(struct trees_istream *sstream)
   i_stream_set_max_buffer_size(sstream->istream.parent,
                                MAX_ISTREAM_BUFFER_SIZE);
 
-  result = trees_istream_read_parent(sstream, MAGIC_SIZE, 0);
+  result = trees_istream_read_parent(sstream, HEADER_SIZE, 0);
   if (result <= 0) {
     /* Make sure we return an error here. */
     result = -1;