diff --git a/rtree/index.py b/rtree/index.py
index d0d96ecc..cb93f8a1 100644
--- a/rtree/index.py
+++ b/rtree/index.py
@@ -196,7 +196,7 @@ def __init__(self,  *args, **kwargs):
             self.properties.filename = basename
 
             # check we can read the file
-            f = basename + "." + self.properties.idx_extension
+            f = basename + "." + self.properties.idx_extension.decode("utf-8")
             p = os.path.abspath(f)