diff --git a/board/sandbox/README.sandbox b/board/sandbox/README.sandbox
index 9fe3bf1..ed820d3 100644
--- a/board/sandbox/README.sandbox
+++ b/board/sandbox/README.sandbox
@@ -44,6 +44,9 @@
       make sandbox_defconfig all NO_SDL=1
       ./u-boot
 
+   If you are building on a 32-bit machine you may get errors from __ffs.h
+   about shifting more than the machine word size. Edit the config file
+   include/configs/sandbox.h and change CONFIG_SANDBOX_BITS_PER_LONG to 32.
 
 U-Boot will start on your computer, showing a sandbox emulation of the serial
 console: