diff --git a/leapdb.c b/leapdb.c
index c5da0ad..80da808 100644
--- a/leapdb.c
+++ b/leapdb.c
@@ -130,7 +130,7 @@ get_list_leap(time_t when, int *tai_offset)
     char *p;
 
     /* Ignore blank lines */
-    for (p = line; *p && isspace(*p); ++p)
+    for (p = line; *p && isspace((unsigned char)*p); ++p)
       ;
     if (!*p)
       continue;