diff --git a/bob/devtools/build.py b/bob/devtools/build.py
index 101379960fdb1a9c4608dfb01259500849de98a1..35a184756afd9ab77720b07784fa519de6f6e267 100644
--- a/bob/devtools/build.py
+++ b/bob/devtools/build.py
@@ -256,7 +256,7 @@ def exists_on_channel(channel_url, basename):
         candidate = urls[other_build_numbers.index(self_build_number)]
         pkg_type = '.conda' if basename.endswith('.conda') else '.tar.bz2'
         if candidate.endswith(pkg_type):  #match
-            return "".join(channel_url, candidate)
+            return "".join((channel_url, candidate))
 
 
 def remove_pins(deps):