diff --git a/src/services/navigationBar.ts b/src/services/navigationBar.ts
index b4c7c5a2c24..f23fa4c3493 100644
--- a/src/services/navigationBar.ts
+++ b/src/services/navigationBar.ts
@@ -612,7 +612,7 @@ namespace ts.NavigationBar {
      * We store 'A' as associated with a NavNode, and use getModuleName to traverse down again.
      */
     function getInteriorModule(decl: ModuleDeclaration): ModuleDeclaration {
-        return decl.body!.kind === SyntaxKind.ModuleDeclaration ? getInteriorModule(<ModuleDeclaration>decl.body) : decl; // TODO: GH#18217
+        return decl.body && isModuleDeclaration(decl.body) ? getInteriorModule(decl.body) : decl;
     }
 
     function isComputedProperty(member: EnumMember): boolean {
diff --git a/tests/cases/fourslash/navigationBarItemsModules.ts b/tests/cases/fourslash/navigationBarItemsModules.ts
index fd5907e2bc7..5fe856c8979 100644
--- a/tests/cases/fourslash/navigationBarItemsModules.ts
+++ b/tests/cases/fourslash/navigationBarItemsModules.ts
@@ -4,6 +4,8 @@
 ////
 ////declare module 'X2.Y2.Z2' {}
 ////
+////declare module "foo";
+////
 ////module A.B.C {
 ////    export var x;
 ////}
@@ -24,9 +26,6 @@
 ////    }
 ////}
 
-//We have 8 module keywords, and 4 var keywords.
-//The declarations of A.B.C.x do not get merged, so the 4 vars are independent.
-//The two 'A' modules, however, do get merged, so in reality we have 7 modules.
 verify.navigationTree({
     "text": "<global>",
     "kind": "script",
@@ -36,6 +35,11 @@ verify.navigationTree({
             "kind": "module",
             "kindModifiers": "declare"
         },
+        {
+            "text": "\"foo\"",
+            "kind": "module",
+            "kindModifiers": "declare"
+        },
         {
             "text": "\"X.Y.Z\"",
             "kind": "module",
@@ -104,6 +108,11 @@ verify.navigationBar([
                 "kind": "module",
                 "kindModifiers": "declare"
             },
+            {
+              "text": "\"foo\"",
+              "kind": "module",
+              "kindModifiers": "declare"
+            },
             {
                 "text": "\"X.Y.Z\"",
                 "kind": "module",
@@ -129,6 +138,12 @@ verify.navigationBar([
         "kindModifiers": "declare",
         "indent": 1
     },
+    {
+        "text": "\"foo\"",
+        "kind": "module",
+        "kindModifiers": "declare",
+        "indent": 1
+    },
     {
         "text": "\"X.Y.Z\"",
         "kind": "module",