diff --git a/packages/compiler-cli/src/ngtsc/reflection/src/typescript.ts b/packages/compiler-cli/src/ngtsc/reflection/src/typescript.ts
index a08a088a68ccb..7c3ebc6e56046 100644
--- a/packages/compiler-cli/src/ngtsc/reflection/src/typescript.ts
+++ b/packages/compiler-cli/src/ngtsc/reflection/src/typescript.ts
@@ -548,6 +548,8 @@ export function reflectTypeEntityToDeclaration(
         throw new Error(`Module specifier is not a string`);
       }
       return {node, from: importDecl.moduleSpecifier.text};
+    } else if (ts.isModuleDeclaration(decl)) {
+      return {node, from: null};
     } else {
       throw new Error(`Unknown import type?`);
     }
diff --git a/packages/compiler-cli/src/ngtsc/reflection/test/ts_host_spec.ts b/packages/compiler-cli/src/ngtsc/reflection/test/ts_host_spec.ts
index d298d2441cd3a..fd5c7c7ffb135 100644
--- a/packages/compiler-cli/src/ngtsc/reflection/test/ts_host_spec.ts
+++ b/packages/compiler-cli/src/ngtsc/reflection/test/ts_host_spec.ts
@@ -153,6 +153,30 @@ runInEachFileSystem(() => {
         expectParameter(args[0], 'bar', {moduleName: './bar', name: 'Bar'});
       });
 
+      it('should reflect an argument from a namespace declarations', () => {
+        const {program} = makeProgram([{
+          name: _('/entry.ts'),
+          contents: `
+            export declare class Bar {}
+            declare namespace i1 {
+              export {
+                Bar,
+              }
+            }
+
+            class Foo {
+              constructor(bar: i1.Bar) {}
+            }
+        `
+        }]);
+        const clazz = getDeclaration(program, _('/entry.ts'), 'Foo', isNamedClassDeclaration);
+        const checker = program.getTypeChecker();
+        const host = new TypeScriptReflectionHost(checker);
+        const args = host.getConstructorParameters(clazz)!;
+        expect(args.length).toBe(1);
+        expectParameter(args[0], 'bar', 'i1.Bar');
+      });
+
       it('should reflect an argument from a default import', () => {
         const {program} = makeProgram([
           {