diff --git a/include/proto/h1.h b/include/proto/h1.h
index b6c61352e..c82e17ad5 100644
--- a/include/proto/h1.h
+++ b/include/proto/h1.h
@@ -309,7 +309,7 @@ static inline struct h1m *h1m_init_req(struct h1m *h1m)
 	h1m->flags = H1_MF_NONE;
 	h1m->curr_len = 0;
 	h1m->body_len = 0;
-	h1m->err_pos = 0;
+	h1m->err_pos = -2;
 	h1m->err_state = 0;
 	return h1m;
 }
@@ -323,7 +323,7 @@ static inline struct h1m *h1m_init_res(struct h1m *h1m)
 	h1m->flags = H1_MF_RESP;
 	h1m->curr_len = 0;
 	h1m->body_len = 0;
-	h1m->err_pos = 0;
+	h1m->err_pos = -2;
 	h1m->err_state = 0;
 	return h1m;
 }
diff --git a/include/types/h1.h b/include/types/h1.h
index 81ea3da9f..7edfbd89e 100644
--- a/include/types/h1.h
+++ b/include/types/h1.h
@@ -143,7 +143,12 @@ enum h1m_state {
 #define H1_MF_RESP              0x00000004 // this message is the response message
 
 
-/* basic HTTP/1 message state for use in parsers */
+/* basic HTTP/1 message state for use in parsers. The err_pos field is special,
+ * it is pre-set to a negative value (-1 or -2), and once non-negative it contains
+ * the relative position in the message of the first parse error. -2 is used to tell
+ * the parser that we want to block the invalid message. -1 is used to only perform
+ * a silent capture.
+ */
 struct h1m {
 	enum h1m_state state;       // H1 message state (H1_MSG_*)
 	/* 8 bits available here */