Implementing a fragment shader that uses a uniform Sampler2D (lwjgl)

Question

I am unable to successfully run a shader, and I seem to be missing some step to make it all work. I end up with the error of:

Exception in thread "main" org.lwjgl.opengl.OpenGLException: Invalid operation (1282)
    at org.lwjgl.opengl.Util.checkGLError(Util.java:59)
    at org.lwjgl.opengl.GL20.glUniform1i(GL20.java:374)
    at sprites.Sprite.draw(Sprite.java:256)
    at gui.Game.drawFrame(Game.java:238)
    at gui.Game.gameLoop(Game.java:205)
    at gui.Game.startGame(Game.java:244)
    at tests.simple.SimpleShader.main(SimpleShader.java:36)

My initialization begins with:

int frag = FilterLoader.createShader("/tests/resources/shaders/grayscale.frag", GL20.GL_FRAGMENT_SHADER);

and the createShader method looks like the following:

int shader = GL20.glCreateShader(type);

if(shader == 0)
    return 0;

StringBuilder code = new StringBuilder("");
String line;

try
{
    String path = FilterLoader.class.getResource(filename).getPath();
    BufferedReader reader = new BufferedReader(new FileReader(path));

    while((line = reader.readLine()) != null)
    {
        code.append(line + "\n");
    }
}
catch(Exception e)
{
    e.printStackTrace();
    System.err.println("Error reading in " + type + " shader");
    return 0;
}

GL20.glShaderSource(shader, code);
GL20.glCompileShader(shader);

return shader;

I then attach the shader to the specific Sprite with:

two.addFragmentShader(frag); //two is a Sprite

which is just simply:

fragmentShader = fragment_shader;
GL20.glAttachShader(shader, fragment_shader);
GL20.glLinkProgram(shader);

The int shader has been previously initialized in the Sprites constructor with:

shader = GL20.glCreateProgram();

This was a previous problem, but no longer obviously. Now I get to where the actual error occured, in the Sprites (two in this case) draw method, which looks like so:

if(true)
{
    GL20.glUseProgram(shader);
}

glPushMatrix();

glActiveTexture(GL13.GL_TEXTURE0);
imageData.getTexture().bind();

//The line below is where the error occurs.
GL20.glUniform1i(fragmentShader, GL13.GL_TEXTURE0);

int tx = (int)location.x;
int ty = (int)location.y;
glTranslatef(tx, ty, location.layer);


float texture_X = ((float)which_column/(float)columns);
float texture_Y = ((float)which_row/(float)rows);
float texture_XplusWidth = ((float)(which_column+wide)/(float)columns);
float texture_YplusHeight = ((float)(which_row+tall)/(float)rows);

glBegin(GL_QUADS);
{
    GL11.glTexCoord2f(texture_X, texture_Y);
    glVertex2f(0, 0);

    GL11.glTexCoord2f(texture_X, texture_YplusHeight);
    glVertex2f(0, getHeight());

    GL11.glTexCoord2f(texture_XplusWidth, texture_YplusHeight);
    glVertex2f(getWidth(), getHeight());

    GL11.glTexCoord2f(texture_XplusWidth, texture_Y);
    glVertex2f(getWidth(), 0);
}
glEnd();
GL20.glUseProgram(0);
glPopMatrix();

And the error occurs at this line:

GL20.glUniform1i(fragmentShader, GL13.GL_TEXTURE0);

And for reference my shader:

// simple fragment shader

uniform sampler2D texture;

void main()
{
    vec4 color, texel;
    color = gl_Color;
    texel = texture2DRect(texture, gl_TexCoord[0].xy);
    color *= texel;

    float gray = dot(color.rgb, vec3(0.299, 0.587, 0.144));

    gl_FragColor = vec4(gray, gray, gray, color.a);
}

I've gone through the tutorials, read about the error, and I can't figure out what step I have missed.