kernel compilation error "kernel is not clean, please run 'make mrproper' "

I got this error while build the kernel.
Actually I am creating the binaries in a separate directory (TEGRA_KERNEL_OUT).
 sachin@sachin-ubuntu$ make O=$TEGRA_KERNEL_OUT zImage
  GEN     /home/sachin/projects/tegra/kern_out/Makefile
scripts/kconfig/conf --silentoldconfig Kconfig
  Using /home/sachin/projects/tegra as source for kernel
  /home/sachin/projects/tegra is not clean, please run 'make mrproper'
  in the '/home/sachin/projects/tegra' directory.
make[1]: * [prepare3] Error 1**
make: * [sub-make] Error 2**


Even after I have executed following it didn't go away. As there will be scripts/files created in kernel source dir as well as TEGRA_KERNEL_OUT.
make O=$TEGRA_KERNEL_OUT mrproper


So I have to do 2 mrproper
make mrproper
make O=$TEGRA_KERNEL_OUT mrproper

And then it worked fine.