Can't write TIFF files using Libtiff on Mac

Question

I use libtiff3.9.4 for reading and writing TIFF files in c++ on my mac. My project is written to be portable and runs without any issues on both Windows 32-bit og Ubuntu 64-bit. But on the mac the Libtiff function TIFFWriteScanline always fails (it returns != 1). The TIFF file is created, but it does not have any contents.

I have tried both libtiff3.8.2 and libtiff4.0.0beta6 without any luck.

Any ideas why libtiff won't write scanlines on the mac when it works fine on linux?