diff --git a/exprtk.hpp b/exprtk.hpp
index f3a53b4..131de03 100644
--- a/exprtk.hpp
+++ b/exprtk.hpp
@@ -5659,9 +5659,13 @@ namespace exprtk
             {
                n0_e.first = false;
 
-               if (!details::is_variable_node(n0_e.second))
+               if (
+                    !is_variable_node(n0_e.second) &&
+                    !is_string_node  (n0_e.second)
+                  )
                {
                   delete n0_e.second;
+                  n0_e.second = expression_node_ptr(0);
                }
             }
 
@@ -5669,9 +5673,13 @@ namespace exprtk
             {
                n1_e.first = false;
 
-               if (!details::is_variable_node(n1_e.second))
+               if (
+                    !is_variable_node(n1_e.second) &&
+                    !is_string_node  (n1_e.second)
+                  )
                {
                   delete n1_e.second;
+                  n1_e.second = expression_node_ptr(0);
                }
             }
          }
@@ -6387,7 +6395,7 @@ namespace exprtk
 
         ~generic_string_range_node()
          {
-            range_.free();
+            base_range_.free();
 
             if (branch_ && branch_deletable_)
             {
diff --git a/readme.txt b/readme.txt
index 15c3ffe..ec3f607 100644
--- a/readme.txt
+++ b/readme.txt
@@ -426,6 +426,9 @@ of C++ compilers:
 |          | 3. x += y[:i + j] + 'abc'                               |
 |          | 4. x += '0123456789'[2:7]                               |
 +----------+---------------------------------------------------------+
+| <=>      | Swap the values of x and y. Where x and y are mutable   |
+|          | strings.  (eg: x <=> y)                                 |
++----------+---------------------------------------------------------+
 | []       | The string size operator returns the size of the string |
 |          | being actioned.                                         |
 |          | eg:                                                     |