diff --git a/lib/6to5/transformation/transformers/other/use-strict.js b/lib/6to5/transformation/transformers/other/use-strict.js
index 2b8251a41b..3965a463de 100644
--- a/lib/6to5/transformation/transformers/other/use-strict.js
+++ b/lib/6to5/transformation/transformers/other/use-strict.js
@@ -17,5 +17,5 @@ exports.FunctionExpression = function (node, parent, scope, context) {
 };
 
 exports.ThisExpression = function (node, parent, scope, context, file) {
-  throw file.errorWithNode(node, "Top level `this` is not allowed", ReferenceError);
+  throw file.errorWithNode(node, "Top level `this` is `undefined` in strict mode", ReferenceError);
 };
diff --git a/test/fixtures/transformation/use-strict/illegal-this-arrow-function/options.json b/test/fixtures/transformation/use-strict/illegal-this-arrow-function/options.json
index c1e1e00456..3137168db6 100644
--- a/test/fixtures/transformation/use-strict/illegal-this-arrow-function/options.json
+++ b/test/fixtures/transformation/use-strict/illegal-this-arrow-function/options.json
@@ -1,3 +1,3 @@
 {
-  "throws": "Top level `this` is not allowed"
+  "throws": "Top level `this` is `undefined` in strict mode"
 }
diff --git a/test/fixtures/transformation/use-strict/illegal-this-root-call/options.json b/test/fixtures/transformation/use-strict/illegal-this-root-call/options.json
index c1e1e00456..3137168db6 100644
--- a/test/fixtures/transformation/use-strict/illegal-this-root-call/options.json
+++ b/test/fixtures/transformation/use-strict/illegal-this-root-call/options.json
@@ -1,3 +1,3 @@
 {
-  "throws": "Top level `this` is not allowed"
+  "throws": "Top level `this` is `undefined` in strict mode"
 }
diff --git a/test/fixtures/transformation/use-strict/illegal-this-root-declaration/options.json b/test/fixtures/transformation/use-strict/illegal-this-root-declaration/options.json
index c1e1e00456..3137168db6 100644
--- a/test/fixtures/transformation/use-strict/illegal-this-root-declaration/options.json
+++ b/test/fixtures/transformation/use-strict/illegal-this-root-declaration/options.json
@@ -1,3 +1,3 @@
 {
-  "throws": "Top level `this` is not allowed"
+  "throws": "Top level `this` is `undefined` in strict mode"
 }
diff --git a/test/fixtures/transformation/use-strict/illegal-this-root-reference/options.json b/test/fixtures/transformation/use-strict/illegal-this-root-reference/options.json
index c1e1e00456..3137168db6 100644
--- a/test/fixtures/transformation/use-strict/illegal-this-root-reference/options.json
+++ b/test/fixtures/transformation/use-strict/illegal-this-root-reference/options.json
@@ -1,3 +1,3 @@
 {
-  "throws": "Top level `this` is not allowed"
+  "throws": "Top level `this` is `undefined` in strict mode"
 }