diff --git a/lbaselib.c b/lbaselib.c
index e5323423..135738fd 100644
--- a/lbaselib.c
+++ b/lbaselib.c
@@ -1,5 +1,5 @@
 /*
-** $Id: lbaselib.c,v 1.250 2010/09/07 19:38:36 roberto Exp roberto $
+** $Id: lbaselib.c,v 1.251 2010/10/28 15:36:30 roberto Exp roberto $
 ** Basic library
 ** See Copyright Notice in lua.h
 */
@@ -43,6 +43,8 @@ static int luaB_print (lua_State *L) {
 }
 
 
+#define SPACECHARS	" \f\n\r\t\v"
+
 static int luaB_tonumber (lua_State *L) {
   int base = luaL_optint(L, 2, 10);
   if (base == 10) {  /* standard conversion */
@@ -53,17 +55,19 @@ static int luaB_tonumber (lua_State *L) {
     }
   }
   else {
-    const char *s1 = luaL_checkstring(L, 1);
+    size_t l1;
+    const char *s1 = luaL_checklstring(L, 1, &l1);
+    const char *e1 = s1 + l1;
     char *s2;
     unsigned long n;
     int neg = 0;
     luaL_argcheck(L, 2 <= base && base <= 36, 2, "base out of range");
-    while (isspace((unsigned char)(*s1))) s1++;  /* skip initial spaces */
+    s1 += strspn(s1, SPACECHARS);  /* skip initial spaces */
     if (*s1 == '-') { s1++; neg = 1; }
     n = strtoul(s1, &s2, base);
     if (s1 != s2) {  /* at least one valid digit? */
-      while (isspace((unsigned char)(*s2))) s2++;  /* skip trailing spaces */
-      if (*s2 == '\0') {  /* no invalid trailing characters? */
+      s2 += strspn(s2, SPACECHARS);  /* skip trailing spaces */
+      if (s2 == e1) {  /* no invalid trailing characters? */
         lua_pushnumber(L, (neg) ? -(lua_Number)n : (lua_Number)n);
         return 1;
       }