diff --git a/test/test-class-extend.js b/test/test-class-extend.js
index 6a78ac0..6def5f0 100644
--- a/test/test-class-extend.js
+++ b/test/test-class-extend.js
@@ -274,7 +274,7 @@ for ( var i = 0; i < class_count; i++ )
     catch ( e )
     {
         assert.notEqual(
-            e.toString().match( args.length + ' given' ),
+            e.message.match( args.length + ' given' ),
             null,
             "Class invocation should give argument count on error"
         );
diff --git a/test/test-class-name.js b/test/test-class-name.js
index 19fa1b9..c5f00d7 100644
--- a/test/test-class-name.js
+++ b/test/test-class-name.js
@@ -66,7 +66,7 @@ var common = require( './common' ),
     catch ( e )
     {
         assert.notEqual(
-            e.toString().match( name ),
+            e.message.match( name ),
             null,
             "Class definition argument count error string contains class name"
         );
@@ -96,7 +96,7 @@ var common = require( './common' ),
     }
     catch ( e )
     {
-        var errstr = e.toString();
+        var errstr = e.message;
 
         assert.notEqual(
             errstr.match( name ),
@@ -256,7 +256,7 @@ var common = require( './common' ),
     catch ( e )
     {
         assert.notEqual(
-            e.toString().match( name ),
+            e.message.match( name ),
             null,
             "Abstract class instantiation error should contain class name"
         );
@@ -276,7 +276,7 @@ var common = require( './common' ),
     catch ( e )
     {
         assert.notEqual(
-            e.toString().match( '(anonymous)' ),
+            e.message.match( '(anonymous)' ),
             null,
             "Abstract class instantiation error should recognize that class " +
                 "is anonymous if no name was given"
diff --git a/test/test-interface-extend.js b/test/test-interface-extend.js
index 2e542c7..f9197c5 100644
--- a/test/test-interface-extend.js
+++ b/test/test-interface-extend.js
@@ -192,7 +192,7 @@ for ( var i = 0; i < base_types.length; i++ )
     catch ( e )
     {
         assert.notEqual(
-            e.toString().match( args.length + ' given' ),
+            e.message.match( args.length + ' given' ),
             null,
             "Interface invocation should give argument count on error"
         );
diff --git a/test/test-interface-name.js b/test/test-interface-name.js
index dad582c..f41f874 100644
--- a/test/test-interface-name.js
+++ b/test/test-interface-name.js
@@ -67,7 +67,7 @@ var common     = require( './common' ),
     catch ( e )
     {
         assert.notEqual(
-            e.toString().match( name ),
+            e.message.match( name ),
             null,
             "Interface definition argument count error string contains " +
                 "interface name"
@@ -98,7 +98,7 @@ var common     = require( './common' ),
     }
     catch ( e )
     {
-        var errstr = e.toString();
+        var errstr = e.message;
 
         assert.notEqual(
             errstr.match( name ),
@@ -210,7 +210,7 @@ var common     = require( './common' ),
         {
             // ensure the error string contains the interface name
             assert.notEqual(
-                e.toString().match( name ),
+                e.message.match( name ),
                 null,
                 "Error contains interface name when available (" + i + ")"
             );
@@ -234,7 +234,7 @@ var common     = require( './common' ),
     catch ( e )
     {
         assert.notEqual(
-            e.toString().match( name ),
+            e.message.match( name ),
             null,
             "Interface name is included in instantiation error message"
         );