Xcode OpenGL ES: Framebuffer is not complete - ios

Question

I am trying to render an image to a texture in OpenGL ES to implement off-screen Sobel filter. Following this link http://www.songho.ca/opengl/gl_fbo.html, I create a resulting texture named sobelTexture and a render frame buffer named depthBuffer, then respectively attach them to GL_COLOR_ATTACHMENT0 and GL_DEPTH_ATTACHMENT of a newly created frame buffer. The code is shown in bellow.
// Create a texture associated with the frame buffer to render image into
GLuint sobelTexture;
glGenTextures(1, &sobelTexture);
glBindTexture(GL_TEXTURE_2D, sobelTexture);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_FLOAT, 0);
glBindTexture(GL_TEXTURE_2D, 0);
GLuint depthBuffer;
glGenRenderbuffers(1, &depthBuffer);
glBindRenderbuffer(GL_RENDERBUFFER, depthBuffer);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT16, width, height);
glBindRenderbuffer(GL_RENDERBUFFER, 0);
// Create frame buffer
GLuint frameBuffer;
glGenFramebuffers(1, &frameBuffer);
glBindFramebuffer(GL_FRAMEBUFFER, frameBuffer);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, sobelTexture, 0);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthBuffer);
However, when I check the frame buffer status using the bellow code, it returns what is different from GL_FRAMEBUFFER_COMPLETE.
if (glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE)
{
NSLog(#"! ERROR: frame buffer is not completed\n");
exit(1);
}
Moreover, If I change the data type GL_FLOAT in glTexImage2D() function to GL_UNSIGNED_BYTE, the frame buffer will be marked as complete. But I cannot use it because, in my fragment shader, the output color components are of float type.
How could I fix this problem?
Thanks in advance for any help.