diff --git a/usr.bin/calendar/io.c b/usr.bin/calendar/io.c
index 42abca62b9b9..d12783f474c6 100644
--- a/usr.bin/calendar/io.c
+++ b/usr.bin/calendar/io.c
@@ -155,8 +155,8 @@ cal(void)
 		}
 
 		/* Get rid of leading spaces (non-standard) */
-		while (isspace(buf[0]))
-			memcpy(buf, buf + 1, strlen(buf) - 1);
+		while (isspace((unsigned char)buf[0]))
+			memcpy(buf, buf + 1, strlen(buf));
 
 		/* No tab in the line, then not a valid line */
 		if ((pp = strchr(buf, '\t')) == NULL)