Why eglGetDisplay(EGL_DEFAULT_DISPLAY) returns EGL_NO_DISPLAY?

[TAEHO NAM]

Hi, I'm new to dri-devel and sorry if this question doesn't fit here.
In my code, the case 1 of getting eglDisplay using EGL_DEFAULT_DISPLAY
failed, but, the case 2 succeeded.

// Case 1
egl_.display = eglGetDisplay (EGL_DEFAULT_DISPLAY);

// Case 2
struct gbm_device * gbm_device_;
gbm_device_ = gbm_create_device (drm _-> GetFD ());
egl_.display = eglGetDisplay ((EGLNativeDisplayType) gbm_device_);

I don't know if EGL_DEFAULT_DISPLAY is missing or it's a problem with my
machine.
The output of lspci is :

VGA compatible controller: Intel Corporation Device 5906 (rev 02)

Thank you for any comments or links to related documents.
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