diff --git a/src/sdtui.c b/src/sdtui.c
index 4961928..8456fb2 100644
--- a/src/sdtui.c
+++ b/src/sdtui.c
@@ -1350,10 +1350,8 @@ on_watch_primary_selection (G_GNUC_UNUSED const gchar *option_name,
 		return TRUE;
 	}
 
-	char *end;
-	errno = 0;
-	long timer = strtol (value, &end, 10);
-	if (errno || *end || end == value || timer <= 0 || timer > G_MAXINT)
+	unsigned long timer;
+	if (!xstrtoul (&timer, value, 10) || !timer || timer > G_MAXINT)
 	{
 		g_set_error (error, G_OPTION_ERROR, G_OPTION_ERROR_FAILED,
 			_("Invalid timer value"));
diff --git a/src/utils.c b/src/utils.c
index b7aa5da..7af0c2f 100644
--- a/src/utils.c
+++ b/src/utils.c
@@ -1,7 +1,7 @@
 /*
  * utils.c: miscellaneous utilities
  *
- * Copyright (c) 2013, Přemysl Janouch <p.janouch@gmail.com>
+ * Copyright (c) 2013 - 2015, Přemysl Janouch <p.janouch@gmail.com>
  * All rights reserved.
  *
  * Permission to use, copy, modify, and/or distribute this software for any
@@ -71,7 +71,7 @@ stream_read_string (GDataInputStream *dis, GError **error)
 	return s;
 }
 
-static bool
+gboolean
 xstrtoul (unsigned long *out, const char *s, int base)
 {
 	char *end;
diff --git a/src/utils.h b/src/utils.h
index c3bdd84..8e43995 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -1,7 +1,7 @@
 /*
  * utils.h: miscellaneous utilities
  *
- * Copyright (c) 2013, Přemysl Janouch <p.janouch@gmail.com>
+ * Copyright (c) 2013 - 2015, Přemysl Janouch <p.janouch@gmail.com>
  * All rights reserved.
  *
  * Permission to use, copy, modify, and/or distribute this software for any
@@ -40,6 +40,7 @@
 
 gboolean stream_read_all (GByteArray *ba, GInputStream *is, GError **error);
 gchar *stream_read_string (GDataInputStream *dis, GError **error);
+gboolean xstrtoul (unsigned long *out, const char *s, int base);
 void update_curses_terminal_size (void);
 
 #endif /* ! UTILS_H */