diff --git a/configure.ac b/configure.ac
index 569d72503..5b5583f9e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1330,6 +1330,7 @@ else
         PYTHON3_INSTALL_DIR="`$PYTHON3 $srcdir/config/config_python.py archsitelib`"
         save_CPPFLAGS=$CPPFLAGS
         CPPFLAGS="$CPPFLAGS $PYTHON3_INCLUDES"
+        AS_UNSET(ac_cv_header_Python_h)
         AC_CHECK_HEADER(Python.h,,[
           use_python3="No (missing header)"
           PYTHON3=