glsl顶点着色器glGetUniformLocation失败

我想在我的顶点着色器中设置一个统一的矢量.

int loc = glGetUniformLocation(shader, "LightPos");
if (loc != -1)
{
    //do Stuff
}

问题是loc始终为-1.我尝试使用片段着色器中的变量实际工作.顶点着色器:

uniform vec3 LightPos;
varying vec2 UVCoord;
varying float LightIntensity;

void main()
{           
    UVCoord = gl_MultiTexCoord0.st;
    gl_Position = ftransform();
    vec3 Normal = normalize(gl_NormalMatrix * gl_Normal);
    LightIntensity = max(dot(normalize(vec3(0, -10, 0)), Normal), 0.0);
}

片段着色器:

uniform sampler2D tex1;
varying vec2 UVCoord;
varying float LightIntensity;

void main()
{
    vec3 Color = vec3(texture2D(tex1, UVCoord));
    gl_FragColor = vec4(Color * LightIntensity, 1.0);
}

有人知道我做错了什么吗？