diff --git a/src/io.c b/src/io.c
index 2bd0ce4..f35e98f 100644
--- a/src/io.c
+++ b/src/io.c
@@ -1885,6 +1885,7 @@ ical_readline (FILE *fdi, char *buf, char *lstore, unsigned *ln)
       if (*lstore != SPACE && *lstore != TAB)
         break;
       strncat (buf, lstore + 1, BUFSIZ);
+      buf[BUFSIZ - 1] = '\0';
       (*ln)++;
     }
 
@@ -2392,7 +2393,8 @@ ical_read_event (FILE *fdi, FILE *log, unsigned *noevents, unsigned *noapoints,
   skip_alarm = 0;
   while (ical_readline (fdi, buf, lstore, lineno))
     {
-      memcpy (buf_upper, buf, strlen (buf));
+      strncpy (buf_upper, buf, BUFSIZ);
+      buf_upper[BUFSIZ - 1] = '\0';
       str_toupper (buf_upper);
 
       if (skip_alarm)
@@ -2576,7 +2578,8 @@ ical_read_todo (FILE *fdi, FILE *log, unsigned *notodos, unsigned *noskipped,
   skip_alarm = 0;
   while (ical_readline (fdi, buf, lstore, lineno))
     {
-      memcpy (buf_upper, buf, strlen (buf));
+      strncpy (buf_upper, buf, BUFSIZ);
+      buf_upper[BUFSIZ - 1] = '\0';
       str_toupper (buf_upper);
       if (skip_alarm)
         {