diff --git a/contrib/tsearch2/stopword.c b/contrib/tsearch2/stopword.c
index 2165bb68dd..5f6d56bce3 100644
--- a/contrib/tsearch2/stopword.c
+++ b/contrib/tsearch2/stopword.c
@@ -48,7 +48,7 @@ readstoplist(text *in, StopList * s)
 		while (fgets(buf, sizeof(buf), hin))
 		{
 			pbuf = buf;
-			while( *pbuf && !isspace( *pbuf ) )
+			while( *pbuf && !isspace((unsigned char) *pbuf ) )
 				pbuf++;
 			*pbuf = '\0';