I have the naive blur convolution shader implementation:

uniform sampler2DRect tex;

void main(void)
{
    vec3 color11 = texture2DRect(tex, gl_TexCoord[0].st +
                                 vec2(-1.0, -1.0)).rgb;

    vec3 color12 = texture2DRect(tex, gl_TexCoord[0].st +
                                 vec2(0.0, -1.0)).rgb;

    vec3 color13 = texture2DRect(tex, gl_TexCoord[0].st +
                                 vec2(1.0, -1.0)).rgb;

    vec3 color21 = texture2DRect(tex, gl_TexCoord[0].st +
                                 vec2(-1.0, 0.0)).rgb;

    vec3 color22 = texture2DRect(tex, gl_TexCoord[0].st).rgb;

    vec3 color23 = texture2DRect(tex, gl_TexCoord[0].st + 
                                 vec2(1.0, 0.0)).rgb;
   
    vec3 color31 = texture2DRect(tex, gl_TexCoord[0].st +
                                 vec2(-1.0, 1.0)).rgb;

    vec3 color32 = texture2DRect(tex, gl_TexCoord[0].st + 
                                 vec2(0.0, 1.0)).rgb;

    vec3 color33 = texture2DRect(tex, gl_TexCoord[0].st +
                                 vec2(1.0, 1.0)).rgb;

    vec3 color = (color11 + color12 + color13 +
                  color21 + color22 + color23 +
                  color31 + color32 + color33);
            
    gl_FragColor = vec4(color / 9.0, 1);
}

It doesn't work, saying that there are too many texture indirections. But if I assign to vec3 color the value:
(2.0 * color11 + 2.0 * color12 + 2.0 * color13 +
 2.0 * color21 + 2.0 * color22 + 2.0 * color23 +
 2.0 * color31 + 2.0 * color32 + 2.0 * color33);
the shader will work. Is it the bug or the normal behavior?