diff --git a/VERSION.md b/VERSION.md
index 04cc1c9..6644f35 100644
--- a/VERSION.md
+++ b/VERSION.md
@@ -1,5 +1,9 @@
 # Version history
 
+## Unreleased
+
+* Fixed bug in Rhubarb for Spine where processing failed depending on the number of existing animations. See [issue #34](https://github.com/DanielSWolf/rhubarb-lip-sync/issues/34#issuecomment-378198776).
+
 ## Version 1.7.1
 
 * Fixed [issue #34](https://github.com/DanielSWolf/rhubarb-lip-sync/issues/34): Generic error message in Rhubarb for Spine
diff --git a/extras/EsotericSoftwareSpine/src/main/kotlin/com/rhubarb_lip_sync/rhubarb_for_spine/SpineJson.kt b/extras/EsotericSoftwareSpine/src/main/kotlin/com/rhubarb_lip_sync/rhubarb_for_spine/SpineJson.kt
index f59df1e..f87d028 100644
--- a/extras/EsotericSoftwareSpine/src/main/kotlin/com/rhubarb_lip_sync/rhubarb_for_spine/SpineJson.kt
+++ b/extras/EsotericSoftwareSpine/src/main/kotlin/com/rhubarb_lip_sync/rhubarb_for_spine/SpineJson.kt
@@ -96,7 +96,7 @@ class SpineJson(val filePath: Path) {
 	}
 
 	val animationNames = observableSet<String>(
-		json.obj("animations")?.map{ it.key }?.toSet() ?: setOf()
+		json.obj("animations")?.map{ it.key }?.toMutableSet() ?: mutableSetOf()
 	)
 
 	fun createOrUpdateAnimation(mouthCues: List<MouthCue>, eventName: String, animationName: String,