Ошибка при присваивании значения в uniform

// vertex
attribute vec4 a_position;
attribute vec2 a_texCoord;
varying vec2 v_texCoord;

void main(void) {
    gl_Position = a_position;
    v_texCoord = a_texCoord;
}

// fragment
varying vec2 v_texCoord;
uniform sampler2D u_texture;
uniform int winRad;
uniform float ml;
uniform float width;
void main()
{
	vec4 tc = texture2D(u_texture, v_texCoord);
	for (int i=1; i<winRad; i++)
	{
		float shift = float(i) / width;
		tc += texture2D(u_texture, v_texCoord + vec2(shift,0.0));
		tc += texture2D(u_texture, v_texCoord - vec2(shift,0.0));
	}
	tc = tc * ml;
	gl_FragColor = tc;
}

например
glUniform1i(glGetUniformLocation(prg,"u_texture"), 0);