2 files changed, 2 insertions, 0 deletions
diff --git a/src/ldo.js b/src/ldo.js
index 9b845e7..fa59e0b 100644
--- a/src/ldo.js
+++ b/src/ldo.js
@@ -77,6 +77,7 @@ const luaD_precall = function(L, off, nresults) {
                 luaD_hook(L, defs.LUA_HOOKCALL, -1);
             let n = f(L); /* do the actual call */
 
+            assert(typeof n == "number" && n >= 0 && n==n|0, "invalid return value from JS function (expected integer)");
             assert(n < L.top - L.ci.funcOff, "not enough elements in the stack");
 
             luaD_poscall(L, ci, L.top - n, n);
diff --git a/src/llex.js b/src/llex.js
index da15b2f..703db49 100644
--- a/src/llex.js
+++ b/src/llex.js
@@ -500,6 +500,7 @@ const llex = function(ls, seminfo) {
     ls.buff.buffer = [];
 
     for (;;) {
+        assert(typeof ls.current == "number");
         switch (ls.current) {
             case char['\n']: case char['\r']: {  /* line breaks */
                 inclinenumber(ls);