diff --git a/src/backend/utils/adt/json.c b/src/backend/utils/adt/json.c
index 8ab47de..1326d34 100644
--- a/src/backend/utils/adt/json.c
+++ b/src/backend/utils/adt/json.c
@@ -419,7 +419,7 @@ json_lex_string(JsonLexContext *lex)
 	for (s = lex->token_start + 1; *s != '"'; ++s)
 	{
 		/* Per RFC4627, these characters MUST be escaped. */
-		if (*s < 32)
+		if ((unsigned char) *s < 32)
 		{
 			/* A NUL byte marks the (premature) end of the string. */
 			if (*s == '\0')
@@ -430,8 +430,8 @@ json_lex_string(JsonLexContext *lex)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
 					 errmsg("invalid input syntax for type json"),
-					 errdetail_internal("line %d: Character \"%c\" must be escaped.",
-						lex->line_number, *s)));
+					 errdetail_internal("line %d: Character with value \"0x%02x\" must be escaped.",
+						lex->line_number, (unsigned char) *s)));
 		}
 		else if (*s == '\\')
 		{
diff --git a/src/test/regress/expected/json.out b/src/test/regress/expected/json.out
index ed8b237..4b1ad89 100644
--- a/src/test/regress/expected/json.out
+++ b/src/test/regress/expected/json.out
@@ -26,8 +26,7 @@ def"'::json;					-- ERROR, unescaped newline in string constant
 ERROR:  invalid input syntax for type json
 LINE 1: SELECT '"abc
                ^
-DETAIL:  line 1: Character "
-" must be escaped.
+DETAIL:  line 1: Character with value "0x0a" must be escaped.
 SELECT '"\n\"\\"'::json;		-- OK, legal escapes
    json   
 ----------