diff --git a/tag.c b/tag.c
index 9d6e040..70dbdcf 100644
--- a/tag.c
+++ b/tag.c
@@ -126,6 +126,9 @@ int_refresh_in(const struct tag *tag, long units)
     if (tag->owner == NULL || tag->owner->refresh_in == NULL)
         return false;
 
+    assert(priv->value_as_int.realtime_unit == TAG_REALTIME_SECS ||
+           priv->value_as_int.realtime_unit == TAG_REALTIME_MSECS);
+
     long milli_seconds = units;
     if (priv->value_as_int.realtime_unit == TAG_REALTIME_SECS)
         milli_seconds *= 1000;
diff --git a/tag.h b/tag.h
index 887cb67..8b4a1f9 100644
--- a/tag.h
+++ b/tag.h
@@ -7,6 +7,7 @@
 enum tag_realtime_unit {
     TAG_REALTIME_NONE,
     TAG_REALTIME_SECS,
+    TAG_REALTIME_MSECS,
 };
 
 struct module;