diff --git a/jsonata.d.ts b/jsonata.d.ts
index 257aac0..0422e22 100644
--- a/jsonata.d.ts
+++ b/jsonata.d.ts
@@ -31,8 +31,8 @@ declare namespace jsonata {
   }
 
   interface Environment {
-    bind(name: string, value: any): void;
-    lookup(name: string): any;
+    bind(name: string | symbol, value: any): void;
+    lookup(name: string | symbol): any;
     readonly timestamp: Date;
     readonly async: boolean;
   }
diff --git a/src/jsonata.js b/src/jsonata.js
index b7dfad5..19e7964 100644
--- a/src/jsonata.js
+++ b/src/jsonata.js
@@ -50,7 +50,7 @@ var jsonata = (function() {
     async function evaluate(expr, input, environment) {
         var result;
 
-        var entryCallback = environment.lookup('__evaluate_entry');
+        var entryCallback = environment.lookup(Symbol.for('jsonata.__evaluate_entry'));
         if(entryCallback) {
             await entryCallback(expr, input, environment);
         }
@@ -124,7 +124,7 @@ var jsonata = (function() {
             result = await evaluateGroupExpression(expr.group, result, environment);
         }
 
-        var exitCallback = environment.lookup('__evaluate_exit');
+        var exitCallback = environment.lookup(Symbol.for('jsonata.__evaluate_exit'));
         if(exitCallback) {
             await exitCallback(expr, input, environment, result);
         }
diff --git a/test/implementation-tests.js b/test/implementation-tests.js
index 6df7e67..1e162f6 100644
--- a/test/implementation-tests.js
+++ b/test/implementation-tests.js
@@ -1057,11 +1057,11 @@ function timeboxExpression(expr, timeout, maxDepth) {
     };
 
     // register callbacks
-    expr.assign("__evaluate_entry", function() {
+    expr.assign(Symbol.for('jsonata.__evaluate_entry'), function() {
         depth++;
         checkRunnaway();
     });
-    expr.assign("__evaluate_exit", function() {
+    expr.assign(Symbol.for('jsonata.__evaluate_exit'), function() {
         depth--;
         checkRunnaway();
     });
diff --git a/test/run-test-suite.js b/test/run-test-suite.js
index d851fbf..60beee0 100644
--- a/test/run-test-suite.js
+++ b/test/run-test-suite.js
@@ -178,12 +178,12 @@ function timeboxExpression(expr, timeout, maxDepth) {
     };
 
     // register callbacks
-    expr.assign("__evaluate_entry", function(expr, input, env) {
+    expr.assign(Symbol.for('jsonata.__evaluate_entry'), function(expr, input, env) {
         if (env.isParallelCall) return;
         depth++;
         checkRunnaway();
     });
-    expr.assign("__evaluate_exit", function(expr, input, env) {
+    expr.assign(Symbol.for('jsonata.__evaluate_exit'), function(expr, input, env) {
         if (env.isParallelCall) return;
         depth--;
         checkRunnaway();