diff --git a/chapter08/grub.xml b/chapter08/grub.xml
index 78304a3ab..08d2ac175 100644
--- a/chapter08/grub.xml
+++ b/chapter08/grub.xml
@@ -50,6 +50,17 @@
       </para>
     </note>
 
+    <warning>
+      <para>Unset any environment variables which may affect the build:</para>
+
+      <screen><userinput remap='pre'>unset {C,CPP,CXX,LD}FLAGS</userinput></screen>
+
+      <para>Don't try <quote>tuning</quote> this package with custom
+      compilation flags: this package is a bootloader, with low-level
+      operations in the source code which is likely to be broken by some
+      aggressive optimizations.</para>
+    </warning>
+
     <para>Prepare GRUB for compilation:</para>
 
 <screen><userinput remap="configure">./configure --prefix=/usr          \