Unable to retrieve user tokens using get_currency_balance

Question

I am trying to get user tokens with the following method:

static async getUserTokens(userAccount) {
    console.log("Get tokens for: " + userAccount);
    try {
      const rpc = new JsonRpc("http://127.0.0.1:8888", { fetch });
      const resp = await rpc.get_currency_balance({
        code: "electro.coin",
        account: "bob",
        symbol: "EL"
      });
      //console.log(resp.rows[0]);
      return resp.rows[0];
    } catch (err) {
      console.error(err);
    }
  }

and it returns:

eosjs-jsonrpc.js:99 POST http://127.0.0.1:8888/v1/chain/get_currency_balance 500 (Internal Server Error)
Error: Invalid cast from type 'object_type' to string
    at new RpcError (eosjs-rpcerror.js:23)
    at JsonRpc.eval (eosjs-jsonrpc.js:118)
    at step (eosjs-jsonrpc.js:36)
    at Object.eval [as next] (eosjs-jsonrpc.js:17)
    at fulfilled (eosjs-jsonrpc.js:8)

Any explanation what am I missing? The token contract is electro.coin, the user is bob and the symbol is EL. I hard-coded the elements for better debugging, but no luck so far...