diff --git a/lapi.c b/lapi.c
index f8f70cd..3467891 100644
--- a/lapi.c
+++ b/lapi.c
@@ -53,6 +53,10 @@
 #define isupvalue(i)		((i) < LUA_REGISTRYINDEX)
 
 
+/*
+** Convert an acceptable index to a pointer to its respective value.
+** Non-valid indices return the special nil value 'G(L)->nilvalue'.
+*/
 static TValue *index2value (lua_State *L, int idx) {
   CallInfo *ci = L->ci;
   if (idx > 0) {
@@ -70,22 +74,26 @@
   else {  /* upvalues */
     idx = LUA_REGISTRYINDEX - idx;
     api_check(L, idx <= MAXUPVAL + 1, "upvalue index too large");
-    if (ttislcf(s2v(ci->func)))  /* light C function? */
-      return &G(L)->nilvalue;  /* it has no upvalues */
-    else {
+    if (ttisCclosure(s2v(ci->func))) {  /* C closure? */
       CClosure *func = clCvalue(s2v(ci->func));
       return (idx <= func->nupvalues) ? &func->upvalue[idx-1]
                                       : &G(L)->nilvalue;
     }
+    else {  /* light C function or Lua function (through a hook)?) */
+      api_check(L, ttislcf(s2v(ci->func)), "caller not a C function");
+      return &G(L)->nilvalue;  /* no upvalues */
+    }
   }
 }
 
-
+/*
+** Convert a valid actual index (not a pseudo-index) to its address.
+*/
 static StkId index2stack (lua_State *L, int idx) {
   CallInfo *ci = L->ci;
   if (idx > 0) {
     StkId o = ci->func + idx;
-    api_check(L, o < L->top, "unacceptable index");
+    api_check(L, o < L->top, "invalid index");
     return o;
   }
   else {    /* non-positive index */