[Solved] Sprites turn into little rainbow rectangular

[duketogo]

tk2d ver : 2.5.8.5
Unity ver : 2017 1.0f3

I'm making a prototype with tk2d.

I made 5 tk2dbuttons. The sprites in those are supposed to be changed with clicks.

But with about 2% of possibility, sprites turn into rainbow.

It doesn't happen everytime and I checked all sprites in SpriteCollection. Checked called sprite number from Debug.

SpriteCollection has no vacancy between any numbers.

So called number had the sprite but it didn't displayed.

(ADDED 12 AUG)

I tested it in Older Unity 5.6.0f3, 2D Toolkit 2.5.7

The same symptom occurs.

[duketogo]

It did happen because of blank blocks of the original image file.

I deleted black blocks but sprite ID still existed.

[unikronsoftware]

Just to clarify what is going on -

Sprite ids are never deleted but they are reused. Until they are a small grid texture is used to identify it as a broken sprite collection - if a sprite still uses the id it appears as the grid texture to be obvious it needs to be fixed.