diff --git a/configure.in b/configure.in
index 5a5ccd0..90684ab 100644
--- a/configure.in
+++ b/configure.in
@@ -3413,6 +3413,7 @@
 ucs4) unicode_size="4"
       AC_DEFINE(Py_UNICODE_SIZE,4)
       ;;
+*) AC_MSG_ERROR([invalid value for --enable-unicode. Use either ucs2 or ucs4 (lowercase).]) ;;
 esac
 
 AH_TEMPLATE(PY_UNICODE_TYPE,