diff --git a/gunicorn/util.py b/gunicorn/util.py
index e88c9c37..e919d53c 100644
--- a/gunicorn/util.py
+++ b/gunicorn/util.py
@@ -126,8 +126,8 @@ def load_class(uri, default="sync", section="gunicorn.workers"):
                 return pkg_resources.load_entry_point("gunicorn",
                             section, uri)
             except ImportError, e:
-                raise RuntimeError("class uri invalid or not found: [%s]",
-                        str(e))
+                raise RuntimeError("class uri invalid or not found: " +
+                        "[%s]" % str(e))
         klass = components.pop(-1)
         mod = __import__('.'.join(components))
         for comp in components[1:]: