diff --git a/src/mapping.ts b/src/mapping.ts
index 1af79d29..2de3a9f7 100644
--- a/src/mapping.ts
+++ b/src/mapping.ts
@@ -222,6 +222,8 @@ function handleDeclarationReflection(decl: DeclarationReflection,
   let empty = true;
   // first check if there are any index signatures, so for example `[name: string]: Foo`
   if (typeof decl.indexSignature !== 'undefined' && typeof decl.indexSignature.parameters !== 'undefined') {
+    out.dynamic = true;
+
     for (const param of decl.indexSignature.parameters) {
       empty = false;
       const template: ElasticsearchDynamicTemplate = {};