diff --git a/configure.in b/configure.in
index 1d8fed5810..8c50ef511e 100644
--- a/configure.in
+++ b/configure.in
@@ -125,12 +125,14 @@ AC_DEFUN([RUBY_NACL],
 ])])
 
 AC_DEFUN([RUBY_NACL_CHECK_PEPPER_TYPES],
-[
+[AS_CASE(["${host_os}"],
+[nacl], [
  AC_CHECK_TYPES([struct PPB_Core, struct PPB_Messaging, struct PPB_Var,
               struct PPB_URLLoader, struct PPB_URLRequestInfo,
               struct PPB_URLResponseInfo, struct PPB_FileRef,
               struct PPP_Instance])
 ])
+])
 
 AC_DEFUN([RUBY_CPPOUTFILE],
 [AC_CACHE_CHECK(whether ${CPP} accepts -o, rb_cv_cppoutfile,