diff --git a/expat/lib/xmlrole.c b/expat/lib/xmlrole.c
index 18d7a51a..c809ee51 100644
--- a/expat/lib/xmlrole.c
+++ b/expat/lib/xmlrole.c
@@ -1292,6 +1292,26 @@ declClose(PROLOG_STATE *state,
   return common(state, tok);
 }
 
+/* This function will only be invoked if the internal logic of the
+ * parser has broken down.  It is used in two cases:
+ *
+ * 1: When the XML prolog has been finished.  At this point the
+ * processor (the parser level above these role handlers) should
+ * switch from prologProcessor to contentProcessor and reinitialise
+ * the handler function.
+ *
+ * 2: When an error has been detected (via common() below).  At this
+ * point again the processor should be switched to errorProcessor,
+ * which will never call a handler.
+ *
+ * The result of this is that error() can only be called if the
+ * processor switch failed to happen, which is an internal error and
+ * therefore we shouldn't be able to provoke it simply by using the
+ * library.  It is a necessary backstop, however, so we merely exclude
+ * it from the coverage statistics.
+ *
+ * LCOV_EXCL_START
+ */
 static int PTRCALL
 error(PROLOG_STATE *UNUSED_P(state),
       int UNUSED_P(tok),
@@ -1301,6 +1321,7 @@ error(PROLOG_STATE *UNUSED_P(state),
 {
   return XML_ROLE_NONE;
 }
+/* LCOV_EXCL_STOP */
 
 static int FASTCALL
 common(PROLOG_STATE *state, int tok)