diff --git a/src/node_contextify.cc b/src/node_contextify.cc
index dc939c6cf98fe3..988de8bb506438 100644
--- a/src/node_contextify.cc
+++ b/src/node_contextify.cc
@@ -414,15 +414,21 @@ class ContextifyContext {
       return;
 
     auto attributes = PropertyAttribute::None;
-    bool is_declared =
-        ctx->global_proxy()->GetRealNamedPropertyAttributes(ctx->context(),
-                                                            property)
+    bool is_declared_on_global_proxy = ctx->global_proxy()
+        ->GetRealNamedPropertyAttributes(ctx->context(), property)
         .To(&attributes);
     bool read_only =
         static_cast<int>(attributes) &
         static_cast<int>(PropertyAttribute::ReadOnly);
 
-    if (is_declared && read_only)
+    bool is_declared_on_sandbox = ctx->sandbox()
+        ->GetRealNamedPropertyAttributes(ctx->context(), property)
+        .To(&attributes);
+    read_only = read_only ||
+        (static_cast<int>(attributes) &
+        static_cast<int>(PropertyAttribute::ReadOnly));
+
+    if (read_only)
       return;
 
     // true for x = 5
@@ -440,10 +446,20 @@ class ContextifyContext {
     // this.f = function() {}, is_contextual_store = false.
     bool is_function = value->IsFunction();
 
+    bool is_declared = is_declared_on_global_proxy || is_declared_on_sandbox;
     if (!is_declared && args.ShouldThrowOnError() && is_contextual_store &&
     !is_function)
       return;
 
+    if (!is_declared_on_global_proxy && is_declared_on_sandbox  &&
+        args.ShouldThrowOnError() && is_contextual_store && !is_function) {
+      // The property exists on the sandbox but not on the global
+      // proxy. Setting it would throw because we are in strict mode.
+      // Don't attempt to set it by signaling that the call was
+      // intercepted. Only change the value on the sandbox.
+      args.GetReturnValue().Set(false);
+    }
+
     ctx->sandbox()->Set(property, value);
   }
 
diff --git a/test/known_issues/test-vm-strict-mode.js b/test/known_issues/test-vm-strict-mode.js
deleted file mode 100644
index 9528944732930c..00000000000000
--- a/test/known_issues/test-vm-strict-mode.js
+++ /dev/null
@@ -1,17 +0,0 @@
-'use strict';
-// https://github.com/nodejs/node/issues/12300
-
-require('../common');
-const assert = require('assert');
-const vm = require('vm');
-
-const ctx = vm.createContext({ x: 42 });
-
-// The following line wrongly throws an
-// error because GlobalPropertySetterCallback()
-// does not check if the property exists
-// on the sandbox. It should just set x to 1
-// instead of throwing an error.
-vm.runInContext('"use strict"; x = 1', ctx);
-
-assert.strictEqual(ctx.x, 1);
diff --git a/test/parallel/test-vm-strict-mode.js b/test/parallel/test-vm-strict-mode.js
index f3820d8aead6af..b1b233664dab9b 100644
--- a/test/parallel/test-vm-strict-mode.js
+++ b/test/parallel/test-vm-strict-mode.js
@@ -1,27 +1,14 @@
 'use strict';
+// https://github.com/nodejs/node/issues/12300
+
 require('../common');
 const assert = require('assert');
 const vm = require('vm');
-const ctx = vm.createContext();
-
-// Test strict mode inside a vm script, i.e., using an undefined variable
-// throws a ReferenceError. Also check that variables
-// that are not successfully set in the vm, must not be set
-// on the sandboxed context.
-
-vm.runInContext('w = 1;', ctx);
-assert.strictEqual(1, ctx.w);
-
-assert.throws(function() { vm.runInContext('"use strict"; x = 1;', ctx); },
-              /ReferenceError: x is not defined/);
-assert.strictEqual(undefined, ctx.x);
 
-vm.runInContext('"use strict"; var y = 1;', ctx);
-assert.strictEqual(1, ctx.y);
+const ctx = vm.createContext({ x: 42 });
 
-vm.runInContext('"use strict"; this.z = 1;', ctx);
-assert.strictEqual(1, ctx.z);
+// This might look as if x has not been declared, but x is defined on the
+// sandbox and the assignment should not throw.
+vm.runInContext('"use strict"; x = 1', ctx);
 
-// w has been defined
-vm.runInContext('"use strict"; w = 2;', ctx);
-assert.strictEqual(2, ctx.w);
+assert.strictEqual(ctx.x, 1);