diff --git a/src/http_htx.c b/src/http_htx.c
index a8c87b766..21a5b57f4 100644
--- a/src/http_htx.c
+++ b/src/http_htx.c
@@ -183,6 +183,10 @@ int http_add_header(struct htx *htx, const struct ist n, const struct ist v)
 
 		blk = pblk;
 	}
+
+	if (htx_get_blk_pos(htx, blk) != htx->front)
+		htx_defrag(htx, NULL);
+
 	return 1;
 }
 
diff --git a/src/htx.c b/src/htx.c
index de879260a..24e387eed 100644
--- a/src/htx.c
+++ b/src/htx.c
@@ -841,6 +841,10 @@ struct htx_blk *htx_add_data_before(struct htx *htx, const struct htx_blk *ref,
 			break;
 		blk = pblk;
 	}
+
+	if (htx_get_blk_pos(htx, blk) != htx->front)
+		htx_defrag(htx, NULL);
+
 	return blk;
 }