diff --git a/kafka/producer.c b/kafka/producer.c
index 3bb7375..f3e2cc0 100644
--- a/kafka/producer.c
+++ b/kafka/producer.c
@@ -332,19 +332,19 @@ lua_producer_produce(struct lua_State *L) {
 
     lua_pushstring(L, "key");
     lua_gettable(L, -2 );
+    size_t key_len;
     // rd_kafka will copy key so no need to worry about this cast
-    char *key = (char *)lua_tostring(L, -1);
-    lua_pop(L, 1);
+    char *key = (char *)lua_tolstring(L, -1, &key_len);
 
-    size_t key_len = key != NULL ? strlen(key) : 0;
+    lua_pop(L, 1);
 
     lua_pushstring(L, "value");
     lua_gettable(L, -2 );
+    size_t value_len;
     // rd_kafka will copy value so no need to worry about this cast
-    char *value = (char *)lua_tostring(L, -1);
-    lua_pop(L, 1);
+    char *value = (char *)lua_tolstring(L, -1, &value_len);
 
-    size_t value_len = value != NULL ? strlen(value) : 0;
+    lua_pop(L, 1);
 
     if (key == NULL && value == NULL) {
         int fail = safe_pushstring(L, "producer message must contains non nil key or value");