diff --git a/heka/files/encoder/es-json.toml b/heka/files/encoder/es-json.toml
index de86b21..edf9c60 100644
--- a/heka/files/encoder/es-json.toml
+++ b/heka/files/encoder/es-json.toml
@@ -1,5 +1,5 @@
 [encoder_{{ name }}]
 type = "ESJsonEncoder"
 index = "{{ values.index }}"
-es_index_from_timestamp = {{ values.es_index_from_timestamp }}
+es_index_from_timestamp = "{{ values.es_index_from_timestamp }}"