diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 25fff43bab7..28c9fb25af9 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -578,7 +578,7 @@ namespace ts {
 
         function cloneSymbol(symbol: Symbol): Symbol {
             const result = createSymbol(symbol.flags, symbol.escapedName);
-            result.declarations = symbol.declarations.slice(0);
+            result.declarations = symbol.declarations ? symbol.declarations.slice() : [];
             result.parent = symbol.parent;
             if (symbol.valueDeclaration) result.valueDeclaration = symbol.valueDeclaration;
             if (symbol.constEnumOnlyModule) result.constEnumOnlyModule = true;
diff --git a/tests/baselines/reference/mergedClassWithNamespacePrototype.errors.txt b/tests/baselines/reference/mergedClassWithNamespacePrototype.errors.txt
new file mode 100644
index 00000000000..5c1d4ec5026
--- /dev/null
+++ b/tests/baselines/reference/mergedClassWithNamespacePrototype.errors.txt
@@ -0,0 +1,15 @@
+/b.ts(2,15): error TS2300: Duplicate identifier 'prototype'.
+
+
+==== /a.d.ts (0 errors) ====
+    declare class Foo {}
+    
+==== /b.ts (1 errors) ====
+    declare namespace Foo {
+        namespace prototype {
+                  ~~~~~~~~~
+!!! error TS2300: Duplicate identifier 'prototype'.
+            function f(): void;
+        }
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/mergedClassWithNamespacePrototype.js b/tests/baselines/reference/mergedClassWithNamespacePrototype.js
new file mode 100644
index 00000000000..fbacd3b8baa
--- /dev/null
+++ b/tests/baselines/reference/mergedClassWithNamespacePrototype.js
@@ -0,0 +1,14 @@
+//// [tests/cases/compiler/mergedClassWithNamespacePrototype.ts] ////
+
+//// [a.d.ts]
+declare class Foo {}
+
+//// [b.ts]
+declare namespace Foo {
+    namespace prototype {
+        function f(): void;
+    }
+}
+
+
+//// [b.js]
diff --git a/tests/cases/compiler/mergedClassWithNamespacePrototype.ts b/tests/cases/compiler/mergedClassWithNamespacePrototype.ts
new file mode 100644
index 00000000000..e4b086ffd27
--- /dev/null
+++ b/tests/cases/compiler/mergedClassWithNamespacePrototype.ts
@@ -0,0 +1,9 @@
+// @Filename: /a.d.ts
+declare class Foo {}
+
+// @Filename: /b.ts
+declare namespace Foo {
+    namespace prototype {
+        function f(): void;
+    }
+}