diff --git a/make-includes/xslt_functions.make b/make-includes/xslt_functions.make
index 9894de3..fdcbeed 100755
--- a/make-includes/xslt_functions.make
+++ b/make-includes/xslt_functions.make
@@ -6,6 +6,14 @@
 #
 ################################################################################
 
+
+################################################################################
+#
+# XSLT must be defined as an environment variable. It's an error not to do so.
+#
+XSLT?=$(error The required environment variable XSLT has not been defined. Please set it to either "saxon", "xalan-c" or "xalan-j", to indicate to your preferred XSLT processor)
+
+
 ################################################################################
 #
 # Pass stylesheet parameters as appropriate to the XSLT processor.