diff --git a/m4/pypath.m4 b/m4/pypath.m4
index b029e01b9..66b1970bc 100644
--- a/m4/pypath.m4
+++ b/m4/pypath.m4
@@ -1,5 +1,9 @@
 AC_DEFUN([adl_CHECK_PYTHON],
  [AM_PATH_PYTHON([2.0])
+  case $PYTHON in
+    [[\\/$]]* | ?:[[\\/]]* );;
+    *) AC_MSG_ERROR([The PYTHON variable must be set to an absolute filename.]);;
+  esac
   AC_CACHE_CHECK([for $am_display_PYTHON includes directory],
     [adl_cv_python_inc],
     [adl_cv_python_inc=`$PYTHON -c "import sys; from distutils import sysconfig;]