diff --git a/src/compiler/binder.ts b/src/compiler/binder.ts
index a3a2be8bca3..170364db3fe 100644
--- a/src/compiler/binder.ts
+++ b/src/compiler/binder.ts
@@ -1602,7 +1602,7 @@ namespace ts {
                 if (hasModifier(node, ModifierFlags.Export)) {
                     errorOnFirstToken(node, Diagnostics.export_modifier_cannot_be_applied_to_ambient_modules_and_module_augmentations_since_they_are_always_visible);
                 }
-                if (isExternalModuleAugmentation(node)) {
+                if (isModuleAugmentationExternal(node)) {
                     declareModuleSymbol(node);
                 }
                 else {
@@ -1618,10 +1618,7 @@ namespace ts {
                     }
 
                     const symbol = declareSymbolAndAddToSymbolTable(node, SymbolFlags.ValueModule, SymbolFlags.ValueModuleExcludes);
-
-                    if (pattern) {
-                        (file.patternAmbientModules || (file.patternAmbientModules = [])).push({ pattern, symbol });
-                    }
+                    file.patternAmbientModules = append(file.patternAmbientModules, pattern && { pattern, symbol });
                 }
             }
             else {
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 0a43b9e7520..24eaf97cab0 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -23771,8 +23771,7 @@ namespace ts {
                         // - augmentation for some external module is applied if symbol for augmentation is merged (it was combined with target module).
                         const checkBody = isGlobalAugmentation || (getSymbolOfNode(node).flags & SymbolFlags.Transient);
                         if (checkBody && node.body) {
-                            // body of ambient external module is always a module block
-                            for (const statement of (<ModuleBlock>node.body).statements) {
+                            for (const statement of node.body.statements) {
                                 checkModuleAugmentationElement(statement, isGlobalAugmentation);
                             }
                         }
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index d8efd8d8af3..7d6e58ad9a4 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2093,6 +2093,9 @@ namespace ts {
 
     export type ModuleBody = NamespaceBody | JSDocNamespaceBody;
 
+    /* @internal */
+    export interface AmbientModuleDeclaration extends ModuleDeclaration { body?: ModuleBlock; }
+
     export interface ModuleDeclaration extends DeclarationStatement, JSDocContainer {
         kind: SyntaxKind.ModuleDeclaration;
         parent?: ModuleBody | SourceFile;
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 8108d0f1692..b6099c3959c 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -424,9 +424,8 @@ namespace ts {
         return node.kind === SyntaxKind.VariableDeclaration && node.parent.kind === SyntaxKind.CatchClause;
     }
 
-    export function isAmbientModule(node: Node): boolean {
-        return node && isModuleDeclaration(node) &&
-            (node.name.kind === SyntaxKind.StringLiteral || isGlobalScopeAugmentation(node));
+    export function isAmbientModule(node: Node): node is AmbientModuleDeclaration {
+        return isModuleDeclaration(node) && (node.name.kind === SyntaxKind.StringLiteral || isGlobalScopeAugmentation(node));
     }
 
     export function isModuleWithStringLiteralName(node: Node): node is ModuleDeclaration {
@@ -457,18 +456,19 @@ namespace ts {
         return !!(module.flags & NodeFlags.GlobalAugmentation);
     }
 
-    export function isExternalModuleAugmentation(node: Node): boolean {
+    export function isExternalModuleAugmentation(node: Node): node is AmbientModuleDeclaration {
+        return isAmbientModule(node) && isModuleAugmentationExternal(node);
+    }
+
+    export function isModuleAugmentationExternal(node: AmbientModuleDeclaration) {
         // external module augmentation is a ambient module declaration that is either:
         // - defined in the top level scope and source file is an external module
         // - defined inside ambient module declaration located in the top level scope and source file not an external module
-        if (!node || !isAmbientModule(node)) {
-            return false;
-        }
         switch (node.parent.kind) {
             case SyntaxKind.SourceFile:
-                return isExternalModule(<SourceFile>node.parent);
+                return isExternalModule(node.parent);
             case SyntaxKind.ModuleBlock:
-                return isAmbientModule(node.parent.parent) && !isExternalModule(<SourceFile>node.parent.parent.parent);
+                return isAmbientModule(node.parent.parent) && isSourceFile(node.parent.parent.parent) && !isExternalModule(node.parent.parent.parent);
         }
         return false;
     }
diff --git a/src/services/importTracker.ts b/src/services/importTracker.ts
index 9f61b4ea69d..807506af24e 100644
--- a/src/services/importTracker.ts
+++ b/src/services/importTracker.ts
@@ -63,7 +63,7 @@ namespace ts.FindAllReferences {
             // Module augmentations may use this module's exports without importing it.
             for (const decl of exportingModuleSymbol.declarations) {
                 if (isExternalModuleAugmentation(decl)) {
-                    addIndirectUser(decl as SourceFileLike);
+                    addIndirectUser(decl);
                 }
             }