diff --git a/src/doc/book/ffi.md b/src/doc/book/ffi.md
index fb8896da86d7..5a2ec86c12f5 100644
--- a/src/doc/book/ffi.md
+++ b/src/doc/book/ffi.md
@@ -580,10 +580,10 @@ However, the language provides a workaround.
 
 As a special case, an `enum` is eligible for the "nullable pointer optimization" if it
 contains exactly two variants, one of which contains no data and the other contains
-a single field of one of the non-nullable types listed above. This means it is represented
-as a single pointer, and the non-data variant is represented as the null pointer. This is
-called an "optimization", but unlike other optimizations it is guaranteed to apply to
-eligible types.
+a single field of one of the non-nullable types listed above (or a struct containing such a type).
+This means it is represented as a single pointer, and the non-data variant is represented as a
+null pointer. This is called an "optimization", but unlike other optimizations it is guaranteed
+to apply to eligible types.
 
 The most common type that takes advantage of the nullable pointer optimization is `Option<T>`,
 where `None` corresponds to `null`. So `Option<extern "C" fn(c_int) -> c_int>` is a correct way