Problems with small FBO sizes in shader sources

import QtQuick 2.0

Item
{
    width: 600
    height: 400

    Rectangle {
        width: 10
        height: 10
        id: rect
        gradient: Gradient {
            GradientStop { position: 0; color: "white" }
            GradientStop { position: 1; color: "black" }
        }
    }

    ShaderEffectSource {
        id: shaderSource
        sourceItem: rect
        textureSize: "32x32"
    }

    ShaderEffectItem {
        width: 100
        height: 100
        x: 50
        y: 50
        property variant source: shaderSource

        fragmentShader: "
        uniform lowp sampler2D source;
        varying highp vec2 qt_TexCoord0;
        void main() {
            gl_FragColor = texture2D(source, qt_TexCoord0) * vec4(1, 0, 0, 1);
        }
        "
    }

}