diff --git a/Include/unicodeobject.h b/Include/unicodeobject.h
index 0b93276..99f54c3 100644
--- a/Include/unicodeobject.h
+++ b/Include/unicodeobject.h
@@ -522,7 +522,8 @@
    character conversion when necessary and falls back to memcpy if possible.
 
    Fail if 'to' is smaller than how_many or smaller than len(from)-from_start,
-   or if kind(from[from_start:from_start+how_many]) > kind(to).
+   or if kind(from[from_start:from_start+how_many]) > kind(to), or if to has
+   more than 1 reference.
 
    Return the number of written character, or return -1 and raise an exception
    on error.