diff --git a/traitsui/qt4/ui_panel.py b/traitsui/qt4/ui_panel.py
index eef8cf91b..03aa54823 100644
--- a/traitsui/qt4/ui_panel.py
+++ b/traitsui/qt4/ui_panel.py
@@ -726,7 +726,7 @@ def _evaluate_label_condition(self, conditions, kind):
             method_to_call = getattr(label, method_dict[kind])
             try:
                 cond_value = eval(when, globals(), context)
-                method_to_call(cond_value)
+                method_to_call(bool(cond_value))
             except Exception:
                 # catch errors in the validate_when expression
                 from traitsui.api import raise_to_debug
diff --git a/traitsui/wx/ui_panel.py b/traitsui/wx/ui_panel.py
index 69184a33a..5e599b7d0 100644
--- a/traitsui/wx/ui_panel.py
+++ b/traitsui/wx/ui_panel.py
@@ -732,7 +732,7 @@ def _evaluate_label_condition(self, conditions, kind):
             method_to_call = getattr(label, method_dict[kind])
             try:
                 cond_value = eval(when, globals(), context)
-                method_to_call(cond_value)
+                method_to_call(bool(cond_value))
             except Exception:
                 # catch errors in the validate_when expression
                 from traitsui.api import raise_to_debug