diff --git a/config.m4 b/config.m4
index 63188c2..1e96289 100644
--- a/config.m4
+++ b/config.m4
@@ -38,8 +38,7 @@ AC_DEFUN([ION_BUNDLE], [dnl
   PHP_ADD_INCLUDE(PHP_EXT_SRCDIR([ion])/ion-c/ionc/include)
   PHP_ADD_INCLUDE(PHP_EXT_SRCDIR([ion])/ion-c/decNumber/include)
 
-  PHP_EXPAND_PATH(PHP_EXT_BUILDDIR([ion]), bdir)
-  bdir="$bdir/ion-c/build"
+  bdir="$ext_builddir/ion-c/build"
   if test "$ext_shared" = yes; then
     ION_SHARED_LIBADD="-lionc_static -ldecNumber_static -L$bdir/ionc -L$bdir/decNumber"
   else