diff --git a/contrib/pg_upgrade/controldata.c b/contrib/pg_upgrade/controldata.c
index 2906ccbf8c..13c95a2e2e 100644
--- a/contrib/pg_upgrade/controldata.c
+++ b/contrib/pg_upgrade/controldata.c
@@ -154,7 +154,7 @@ get_control_data(ClusterInfo *cluster, bool live_check)
 		if (GET_MAJOR_VERSION(cluster->major_version) <= 803)
 		{
 			for (p = bufin; *p; p++)
-				if (!isascii(*p))
+				if (!isascii((unsigned char) *p))
 					pg_fatal("The 8.3 cluster's pg_controldata is incapable of outputting ASCII, even\n"
 							 "with LANG=C.  You must upgrade this cluster to a newer version of PostgreSQL\n"
 							 "8.3 to fix this bug.  PostgreSQL 8.3.7 and later are known to work properly.\n");
diff --git a/src/backend/utils/adt/json.c b/src/backend/utils/adt/json.c
index 841dd1aeae..bc0c602fa3 100644
--- a/src/backend/utils/adt/json.c
+++ b/src/backend/utils/adt/json.c
@@ -2353,8 +2353,11 @@ escape_json(StringInfo buf, const char *str)
 				 * only unicode escape that should be present is \u0000,
 				 * all the other unicode escapes will have been resolved.
 				 */
-				if (p[1] == 'u' && isxdigit(p[2]) && isxdigit(p[3])
-					&& isxdigit(p[4]) && isxdigit(p[5]))
+				if (p[1] == 'u' &&
+					isxdigit((unsigned char) p[2]) &&
+					isxdigit((unsigned char) p[3]) &&
+					isxdigit((unsigned char) p[4]) &&
+					isxdigit((unsigned char) p[5]))
 					appendStringInfoCharMacro(buf, *p);
 				else
 					appendStringInfoString(buf, "\\\\");