Electron holography in a field emission gun transmission electron microscope has been used to profile the inner potential V-0 across GaN/x nm In0.1Ga0.9N/GaN/(0001) sapphire samples (x=10. 40nm) grown by molecular beam epitaxy and viewed in cross-section. Results are presented which suggest a decrease in V-0 of 3-4 V across the InGaN layer in the [0001] direction. It is proposed that the results can be explained by charge accumulation across the InGaN layer and that the opposing contributions due to piezoelectric and polarisation fields are effectively masked by Fermi level pinning.