diff --git a/packages/compiler/src/metadata_resolver.ts b/packages/compiler/src/metadata_resolver.ts
index b64968eebe..f9d71b4fab 100644
--- a/packages/compiler/src/metadata_resolver.ts
+++ b/packages/compiler/src/metadata_resolver.ts
@@ -664,16 +664,18 @@ export class CompileMetadataResolver {
   }
 
   private _getTypeDescriptor(type: Type): string {
-    if (this.isDirective(type)) {
-      return 'directive';
-    }
+    if (isValidType(type)) {
+      if (this.isDirective(type)) {
+        return 'directive';
+      }
 
-    if (this.isPipe(type)) {
-      return 'pipe';
-    }
+      if (this.isPipe(type)) {
+        return 'pipe';
+      }
 
-    if (this.isNgModule(type)) {
-      return 'module';
+      if (this.isNgModule(type)) {
+        return 'module';
+      }
     }
 
     if ((type as any).provide) {
diff --git a/packages/compiler/test/metadata_resolver_spec.ts b/packages/compiler/test/metadata_resolver_spec.ts
index df9e3b1c3a..313d95c7d6 100644
--- a/packages/compiler/test/metadata_resolver_spec.ts
+++ b/packages/compiler/test/metadata_resolver_spec.ts
@@ -399,6 +399,18 @@ export function main() {
 
          expect(() => resolver.getNgModuleMetadata(ModuleWithComponentInBootstrap)).not.toThrow();
        }));
+
+    // #20049
+    it('should throw a reasonable message when an invalid import is given',
+       inject([CompileMetadataResolver], (resolver: CompileMetadataResolver) => {
+         @NgModule({imports: [{ngModule: true as any}]})
+         class InvalidModule {
+         }
+
+         expect(() => { resolver.getNgModuleMetadata(InvalidModule); })
+             .toThrowError(
+                 `Unexpected value '[object Object]' imported by the module 'InvalidModule'. Please add a @NgModule annotation.`);
+       }));
   });
 
   it('should dedupe declarations in NgModule',