diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 2639be7f16c..e772c2f0057 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -25929,7 +25929,7 @@ namespace ts {
                 // populate reverse mapping: file path -> type reference directive that was resolved to this file
                 fileToDirective = createMap<string>();
                 resolvedTypeReferenceDirectives.forEach((resolvedDirective, key) => {
-                    if (!resolvedDirective) {
+                    if (!resolvedDirective || !resolvedDirective.resolvedFileName) {
                         return;
                     }
                     const file = host.getSourceFile(resolvedDirective.resolvedFileName);
diff --git a/src/compiler/moduleNameResolver.ts b/src/compiler/moduleNameResolver.ts
index e9f73e9ebfe..a203b2bfaba 100644
--- a/src/compiler/moduleNameResolver.ts
+++ b/src/compiler/moduleNameResolver.ts
@@ -794,6 +794,7 @@ namespace ts {
         if (traceEnabled) {
             trace(host, Diagnostics.Resolving_real_path_for_0_result_1, path, real);
         }
+        Debug.assert(host.fileExists(real), `${path} linked to nonexistent file ${real}`); // tslint:disable-line
         return real;
     }
 
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index d7f14f729f2..801d44e2012 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2779,7 +2779,7 @@ namespace ts {
         getCompilerOptions(): CompilerOptions;
 
         getSourceFiles(): ReadonlyArray<SourceFile>;
-        getSourceFile(fileName: string): SourceFile;
+        getSourceFile(fileName: string): SourceFile | undefined;
         getResolvedTypeReferenceDirectives(): ReadonlyMap<ResolvedTypeReferenceDirective>;
     }