diff --git a/final_walk.cc b/final_walk.cc
index 8e019bc..689061e 100644
--- a/final_walk.cc
+++ b/final_walk.cc
@@ -120,6 +120,27 @@ void LLScriptListConstant::final_pre_checks() {
    }
 }
 
+void LLScriptTypecastExpression::final_pre_checks() {
+   if (!mono_mode && get_type()->get_itype() == LST_LIST) {
+      // Check if it only has a variable of type string or key
+      LLASTNode *child = get_children();
+      if (child && !child->get_next() && child->get_node_type() == NODE_EXPRESSION && child->get_node_sub_type() == NODE_LVALUE_EXPRESSION) {
+         // We're on the right track. Go one level deeper.
+         child = child->get_children();
+         if (child && !child->get_next() && child->get_node_type() == NODE_IDENTIFIER) {
+            LST_TYPE type = child->get_type()->get_itype();
+            // A-ha!
+            if (type == LST_STRING) {
+                ERROR(HERE, W_LSO_LIST_TO_KEY_OR_STR, "string", "key", "string");
+            }  else if (type == LST_KEY) {
+                ERROR(HERE, W_LSO_LIST_TO_KEY_OR_STR, "key", "string", "key");
+            }
+         }
+      }
+
+   }
+}
+
 void LLScriptIfStatement::final_pre_checks() {
    check_cond((LLScriptExpression*)get_child(0), true);
 }
diff --git a/logger.cc b/logger.cc
index 484e0ce..1d1a4a5 100644
--- a/logger.cc
+++ b/logger.cc
@@ -317,4 +317,6 @@ const char *Logger::warning_messages[] = {
    "Prefixing a string with L will cause a double quote (\")\n"
      "to be inserted at the beginning of the string.",                // 20019
    "print does nothing. Attempting to use the result %s.\n",          // 20020
+   "In LSO, (list)%s_var can result in a variable of type %s in the list."
+     " Use [%s_var] instead.",                                      // 20021
 };
diff --git a/logger.hh b/logger.hh
index 4217422..6ae8dd2 100644
--- a/logger.hh
+++ b/logger.hh
@@ -113,6 +113,7 @@ enum ErrorCode {
     W_DUPLICATE_CASE,                  // 20018
     W_L_STRING,                        // 20019
     W_PRINT,                           // 20020
+    W_LSO_LIST_TO_KEY_OR_STR,          // 20021
     W_LAST
 
 };
diff --git a/lslmini.hh b/lslmini.hh
index f15251b..17cda70 100644
--- a/lslmini.hh
+++ b/lslmini.hh
@@ -547,6 +547,7 @@ class LLScriptTypecastExpression : public LLScriptExpression {
     virtual void determine_type() {}; // type already determined
     virtual const char *get_node_name() { return "typecast expression"; }
     virtual LLNodeSubType get_node_sub_type() { return NODE_TYPECAST_EXPRESSION; };
+    virtual void final_pre_checks();
 };
 
 class LLScriptFunctionExpression : public LLScriptExpression {
diff --git a/scripts/lso/liststrkey.lsl b/scripts/lso/liststrkey.lsl
new file mode 100644
index 0000000..f2f407d
--- /dev/null
+++ b/scripts/lso/liststrkey.lsl
@@ -0,0 +1,22 @@
+default
+{
+    state_entry()
+    {
+        key k = llGetObjectName();
+        string s = llGetKey();
+
+        list ls =
+                  (list)    // $[E20021] could result in a string
+                        k;
+        ls = ls +
+                  (list)         // $[E20021] could result in a key
+                        s;
+
+        if (llGetListEntryType(ls, 0) == TYPE_STRING
+         && llGetListEntryType(ls, 1) == TYPE_KEY)
+        {
+            // This is printed!
+            llOwnerSay("LSO has weird bugs");
+        }
+    }
+}
diff --git a/scripts/mono/liststrkey.lsl b/scripts/mono/liststrkey.lsl
new file mode 100644
index 0000000..2fde89d
--- /dev/null
+++ b/scripts/mono/liststrkey.lsl
@@ -0,0 +1,18 @@
+default
+{
+    state_entry()
+    {
+        key k = llGetObjectName();
+        string s = llGetKey();
+
+        list ls = (list)k;  // no E20021 in mono
+        ls = ls + (list)s;  // no E20021 in mono
+
+        if (llGetListEntryType(ls, 0) != TYPE_STRING
+         && llGetListEntryType(ls, 1) != TYPE_KEY)
+        {
+            // Not printed in Mono
+            llOwnerSay("Mono is safe");
+        }
+    }
+}