diff --git a/configure.ac b/configure.ac
index 245af1aa..d554927c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -105,7 +105,7 @@ PKG_CHECK_MODULES([ICU], [icu-i18n icu-uc], [
 	old_LIBS="$LIBS"
 	AC_SUBST(ICU_CFLAGS)
 	AC_SUBST(ICU_LIBS)
-	CPPFLAGS="$CPPFLAGS $ICU_CFLAGS"
+	CPPFLAGS="$CPPFLAGS $ICU_CFLAGS -DU_USING_ICU_NAMESPACE=0"
 	LIBS="$LIBS $ICU_LIBS"
 	AC_MSG_CHECKING([whether boost.regex was compiled with ICU support])
 	AC_LINK_IFELSE([AC_LANG_PROGRAM([