diff --git a/board/sandbox/README.sandbox b/board/sandbox/README.sandbox
index 9fe3bf171a..ed820d338e 100644
--- a/board/sandbox/README.sandbox
+++ b/board/sandbox/README.sandbox
@@ -44,6 +44,9 @@ Note:
       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:
diff --git a/include/configs/sandbox.h b/include/configs/sandbox.h
index 4de89f8879..197d8bb8ca 100644
--- a/include/configs/sandbox.h
+++ b/include/configs/sandbox.h
@@ -27,7 +27,10 @@
 
 #define CONFIG_SYS_STDIO_DEREGISTER
 
-/* Number of bits in a C 'long' on this architecture */
+/*
+ * Number of bits in a C 'long' on this architecture. Set this to 32 when
+ * building on a 32-bit machine.
+ */
 #define CONFIG_SANDBOX_BITS_PER_LONG	64
 
 #define CONFIG_LMB