diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 12dccd8bc64..8ef07866299 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -27980,8 +27980,8 @@ namespace ts {
 
                 // The following checks only apply on a non-ambient instantiated module declaration.
                 if (symbol.flags & SymbolFlags.ValueModule
-                    && symbol.declarations.length > 1
                     && !inAmbientContext
+                    && symbol.declarations.length > 1
                     && isInstantiatedModule(node, !!compilerOptions.preserveConstEnums || !!compilerOptions.isolatedModules)) {
                     const firstNonAmbientClassOrFunc = getFirstNonAmbientClassOrFunctionDeclaration(symbol);
                     if (firstNonAmbientClassOrFunc) {
diff --git a/tests/baselines/reference/extendGlobalThis.js b/tests/baselines/reference/extendGlobalThis.js
new file mode 100644
index 00000000000..c8b73a0c39f
--- /dev/null
+++ b/tests/baselines/reference/extendGlobalThis.js
@@ -0,0 +1,24 @@
+//// [tests/cases/compiler/extendGlobalThis.ts] ////
+
+//// [extension.d.ts]
+declare global {
+    namespace globalThis  {
+        var test: string;
+    }
+}
+
+export {}
+
+//// [index.ts]
+import "./extention";
+
+globalThis.tests = "a-b";
+console.log(globalThis.test.split("-"));
+
+
+//// [index.js]
+"use strict";
+exports.__esModule = true;
+require("./extention");
+globalThis.tests = "a-b";
+console.log(globalThis.test.split("-"));
diff --git a/tests/baselines/reference/extendGlobalThis.symbols b/tests/baselines/reference/extendGlobalThis.symbols
new file mode 100644
index 00000000000..944d7b23715
--- /dev/null
+++ b/tests/baselines/reference/extendGlobalThis.symbols
@@ -0,0 +1,30 @@
+=== tests/cases/compiler/extension.d.ts ===
+declare global {
+>global : Symbol(global, Decl(extension.d.ts, 0, 0))
+
+    namespace globalThis  {
+>globalThis : Symbol(globalThis)
+
+        var test: string;
+>test : Symbol(test, Decl(extension.d.ts, 2, 11))
+    }
+}
+
+export {}
+
+=== tests/cases/compiler/index.ts ===
+import "./extention";
+
+globalThis.tests = "a-b";
+>globalThis : Symbol(globalThis)
+
+console.log(globalThis.test.split("-"));
+>console.log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>console : Symbol(console, Decl(lib.dom.d.ts, --, --))
+>log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>globalThis.test.split : Symbol(String.split, Decl(lib.es5.d.ts, --, --))
+>globalThis.test : Symbol(test, Decl(extension.d.ts, 2, 11))
+>globalThis : Symbol(globalThis)
+>test : Symbol(test, Decl(extension.d.ts, 2, 11))
+>split : Symbol(String.split, Decl(lib.es5.d.ts, --, --))
+
diff --git a/tests/baselines/reference/extendGlobalThis.types b/tests/baselines/reference/extendGlobalThis.types
new file mode 100644
index 00000000000..88fe0462d65
--- /dev/null
+++ b/tests/baselines/reference/extendGlobalThis.types
@@ -0,0 +1,37 @@
+=== tests/cases/compiler/extension.d.ts ===
+declare global {
+>global : typeof global
+
+    namespace globalThis  {
+>globalThis : typeof globalThis
+
+        var test: string;
+>test : string
+    }
+}
+
+export {}
+
+=== tests/cases/compiler/index.ts ===
+import "./extention";
+
+globalThis.tests = "a-b";
+>globalThis.tests = "a-b" : "a-b"
+>globalThis.tests : any
+>globalThis : typeof globalThis
+>tests : any
+>"a-b" : "a-b"
+
+console.log(globalThis.test.split("-"));
+>console.log(globalThis.test.split("-")) : void
+>console.log : (message?: any, ...optionalParams: any[]) => void
+>console : Console
+>log : (message?: any, ...optionalParams: any[]) => void
+>globalThis.test.split("-") : string[]
+>globalThis.test.split : (separator: string | RegExp, limit?: number) => string[]
+>globalThis.test : string
+>globalThis : typeof globalThis
+>test : string
+>split : (separator: string | RegExp, limit?: number) => string[]
+>"-" : "-"
+
diff --git a/tests/cases/compiler/extendGlobalThis.ts b/tests/cases/compiler/extendGlobalThis.ts
new file mode 100644
index 00000000000..1ad9f0552b4
--- /dev/null
+++ b/tests/cases/compiler/extendGlobalThis.ts
@@ -0,0 +1,14 @@
+// @filename: extension.d.ts
+declare global {
+    namespace globalThis  {
+        var test: string;
+    }
+}
+
+export {}
+
+// @filename: index.ts
+import "./extention";
+
+globalThis.tests = "a-b";
+console.log(globalThis.test.split("-"));