diff --git a/scripts/build/transformers/plugin-class.ts b/scripts/build/transformers/plugin-class.ts
index c6f353500..ccc4c56b1 100644
--- a/scripts/build/transformers/plugin-class.ts
+++ b/scripts/build/transformers/plugin-class.ts
@@ -57,7 +57,8 @@ function transformClasses(
   return ts.visitEachChild(
     file,
     node => {
-      if (node.kind !== ts.SyntaxKind.ClassDeclaration) {
+      if (node.kind !== ts.SyntaxKind.ClassDeclaration
+          || (node.modifiers && node.modifiers.find(v => v.kind === ts.SyntaxKind.DeclareKeyword))) {
         return node;
       }
       return transformClass(node, ngcBuild);