diff --git a/pdf.c b/pdf.c
index 8f4fa53f554e3cdf6b23ec64ab81eb273c836ff2..6050dc39de88d9fa711e55f184f61421d9ab5ff7 100644
--- a/pdf.c
+++ b/pdf.c
@@ -751,7 +751,9 @@ act_xrstm(const HParseResult *p, void *u)
 }
 
 /*
- * validate the /Type field on a cross-reference stream.
+ * validate the presence of an AST holding xref entries,
+ * (i. e. a successful parse of an xref table),
+ * and the /Type field on a cross-reference stream
  *
  * p = ((pnat nat) (dict offs offs))
  */