diff --git a/lib/types.js b/lib/types.js
index cba76fd2..1e15acfc 100644
--- a/lib/types.js
+++ b/lib/types.js
@@ -1280,7 +1280,7 @@ function generateDefaultIndexer(types) {
       }
       bucketIndices[bucket] = index;
     }
-  }, this);
+  });
   return (val) => {
     let index = bucketIndices[getValueBucket(val)];
     if (dynamicBranches.length) {
@@ -1319,7 +1319,7 @@ class UnwrappedUnionType extends UnionType {
     }
     this._getIndex = _projectionFn
       ? generateProjectionIndexer(_projectionFn)
-      : generateDefaultIndexer.bind(this)(this.types);
+      : generateDefaultIndexer(this.types);
 
     Object.freeze(this);
   }