diff --git a/libs/pbd/pbd/signals.py b/libs/pbd/pbd/signals.py
index 5691f5a36f..6535ddde2e 100644
--- a/libs/pbd/pbd/signals.py
+++ b/libs/pbd/pbd/signals.py
@@ -275,7 +275,7 @@ def signal(f, n, v):
 \t}
 """, file=f)
     print("""
-\tbool size () const {
+\tsize_t size () const {
 \t\tGlib::Threads::Mutex::Lock lm (_mutex);
 \t\treturn _slots.size ();
 \t}