Ptr byte to cstring?

[Nlits]

I am working with the [freeimage](https://nimble.directory/pkg/freeimage) nim 
library/wrapper, and have this code:
    
    
    proc compressJpeg(imgData: string): string =
      ## Use freeimage to compress the jpeg files
      
      # Open image
      var mem = FreeImage_OpenMemory(cast[ptr byte](imgData.cstring), 
imgData.len.uint32)
      var image = FreeImage_LoadFromMemory(FIF_JPEG, mem, JPEG_ACCURATE)
      FreeImage_CloseMemory(mem)
      
      # Save image
      let FI_DEFAULT = 0.uint8
      var outmem = FreeImage_OpenMemory(addr FI_DEFAULT, FI_DEFAULT.uint32)
      
      doAssert FreeImage_SaveToMemory(FIF_JPEG, image, outmem, 
JPEG_OPTIMIZE).bool
      
      
      # to nim
      var buffer: ptr byte
      var length: uint32
      doAssert FreeImage_AcquireMemory(outmem, addr buffer, addr length).bool
      
      let cs = cast[cstring](buffer)
      
      
      doAssert cs.len.uint32 == length # Ensure converison was sucessful
      result = $cs
      doAssert result.len.uint32 == length # Ensure converison was sucessful
      
      # Cleanup
      FreeImage_Unload(image)
      FreeImage_CloseMemory(outmem)
    
    
    Run

But `doAssert cs.len.uint32 == length` always fires, with cs being 4 bytes 
long? I assume I am converting `buffer` to a cstring in the wrong way, but 
there could be another issue. Is there a better way than cast to convert 
between ptr byte and cstring?