diff --git a/src/VREF/refex-expr b/src/VREF/refex-expr
index 3ff4bf4..d0a51b7 100755
--- a/src/VREF/refex-expr
+++ b/src/VREF/refex-expr
@@ -3,6 +3,8 @@ use strict;
 use warnings;
 
 my $rule = $ARGV[7];
+die "\n\nFATAL: GL_REFEX_EXPR_ doesn't exist\n(your admin probably forgot the rc file change needed for this to work)\n\n"
+  unless exists $ENV{"GL_REFEX_EXPR_" . $rule};
 my $res = $ENV{"GL_REFEX_EXPR_" . $rule} || 0;
 print "$ARGV[6] ($res)\n" if $res;