php mutliple nested if statements

I am having trouble with nested php statements. I am getting the error:

Parse error: syntax error, unexpected 'if' (T_IF)

and it is the commented line below.

Is this because it is nested or because the if statement preceding it needs a colon or semi-colon to finish it?

<?php

    $error = 0;
    $error_note = "";

    if(isset($_GET['carts'])) {

        // Set Carts Variable

        $carts = $_GET['carts'];

        // Check for Valid Numeric Value

        if (is_numeric($carts)) {} else { 
            $error++; 
            $error_note .= "Please enter a valid number.";
        }

        // Set Calculations for Sustainability


        if($error == 0) { //this line

            $NumTruckLoads = ceil($carts / 60);
            $NumOtherTruckLoads = ceil($carts / 20);

            $MPG = $NumTruckLoads * 1.66667;
            $MPGOther = $NumOtherLoads * 1.66667;
        }

    } else {

        $carts = 1000;
        $error++;
    }

?>