diff --git a/spot/tl/hierarchy.cc b/spot/tl/hierarchy.cc
index 3ab6f5930..5112c9c12 100644
--- a/spot/tl/hierarchy.cc
+++ b/spot/tl/hierarchy.cc
@@ -312,7 +312,7 @@ namespace spot
     bool wide = false;
     if (opt)
       for (;;)
-        switch (int o = *opt++)
+        switch (char o = *opt++)
           {
           case 'v':
             verbose = true;