Index: src/Data/Drd/drd_info.cpp
===================================================================
--- src/Data/Drd/drd_info.cpp	(revision 4715)
+++ src/Data/Drd/drd_info.cpp	(working copy)
@@ -643,6 +643,9 @@
 
 tree
 drd_env_read (tree env, string var, tree val) {
+  if (!is_compound(env)) {
+    FAILED("expecting a coumpout env");
+  }
   int i, n= N(env);
   for (i=0; i<n; i+=2)
     if (env[i] == var)