diff --git a/src/bin/pg_basebackup/pg_basebackup.c b/src/bin/pg_basebackup/pg_basebackup.c
index 1e22969d552..56657a42c40 100644
--- a/src/bin/pg_basebackup/pg_basebackup.c
+++ b/src/bin/pg_basebackup/pg_basebackup.c
@@ -1134,7 +1134,7 @@ escapeConnectionParameter(const char *src)
 
 	for (p = src; *p; p++)
 	{
-		if (isspace(*p))
+		if (isspace((unsigned char) *p))
 		{
 			need_quotes = true;
 			break;