diff --git a/dlls/jscript/string.c b/dlls/jscript/string.c
index d74f8cf1e87..62cc6f0cda9 100644
--- a/dlls/jscript/string.c
+++ b/dlls/jscript/string.c
@@ -538,7 +538,7 @@ static HRESULT String_lastIndexOf(script_ctx_t *ctx, vdisp_t *jsthis, WORD flags
         VARIANT *retv, jsexcept_t *ei)
 {
     BSTR search_str, val_str;
-    DWORD length, pos, search_len;
+    DWORD length, pos = 0, search_len;
     const WCHAR *str;
     INT ret = -1;
     HRESULT hres;
@@ -568,15 +568,13 @@ static HRESULT String_lastIndexOf(script_ctx_t *ctx, vdisp_t *jsthis, WORD flags
 
     if(arg_cnt(dp) >= 2) {
         VARIANT ival;
+        double d;
 
         hres = to_integer(ctx, get_arg(dp,1), ei, &ival);
         if(SUCCEEDED(hres)) {
-            if(V_VT(&ival) == VT_I4)
-                pos = V_VT(&ival) > 0 ? V_I4(&ival) : 0;
-            else
-                pos = V_R8(&ival) > 0.0 ? length : 0;
-            if(pos > length)
-                pos = length;
+            d = num_val(&ival);
+            if(d > 0)
+                pos = is_int32(d) ? min((int)d, length) : length;
         }
     }else {
         pos = length;
diff --git a/dlls/jscript/tests/api.js b/dlls/jscript/tests/api.js
index 8d8e819853f..b25f0ae92bb 100644
--- a/dlls/jscript/tests/api.js
+++ b/dlls/jscript/tests/api.js
@@ -496,6 +496,8 @@ tmp = "aaaa".lastIndexOf("a",2);
 ok(tmp == 2, "lastIndexOf = " + tmp);
 tmp = strObj.lastIndexOf("b");
 ok(tmp === 1, "lastIndexOf = " + tmp);
+tmp = "bbb".lastIndexOf("b", bigInt);
+ok(tmp === 2, "lastIndexOf = " + tmp);
 
 tmp = "".toLowerCase();
 ok(tmp === "", "''.toLowerCase() = " + tmp);