libpng is erroneous about png_get_PLTE

I had coded a wrapper for this C implementation in C++ which I recently turned to again. I realized that the function is either erroneous for its wrapper method or for its direct call inside a public method. Why does only one of these two approaches run?

Code:

If (0 !== png_get_PLTE (png,pnginfo,&pltp,&num_plt))
  png_set_PLTE (png,pnginfo,pltp,num_plt);

Or:

this->pngobject.get_PLTE (&pltp,&num_plt);
png_set_PLTE (png,pnginfo,pltp,num_plt);

The first two lines are erroneous for a picture that incorporates these parameters: Height 311, Width 233, Bit Depth 8, Color Type 3, Interlace Type 0, Filter 0, Channels 1, Row Bytes 233; Whereas the API is erroneous by libpng error: Valid palette required for paletted images.

The second approach is erroneous for a picture that incorporates these parameters: Height 600, Width 800, Bit Depth 8, Color Type 6, Interlace Type 0, Filter 0, Channels 4, Row Bytes 3200; Whereas the API is erroneous by libpng error: Invalid palette.

The issue is dumb because of successful calls if the codes read and write the other picture.

So, where is the issue? What is problem at all?