diff --git a/src/emu/dislot.c b/src/emu/dislot.c
index b042755c6bd..058e280b8cf 100644
--- a/src/emu/dislot.c
+++ b/src/emu/dislot.c
@@ -53,7 +53,7 @@ device_t* device_slot_interface::get_card_device()
 	return dev;
 }
 
-const bool device_slot_interface::all_internal()
+const bool device_slot_interface::all_internal() const
 {
 	for (int i = 0; m_slot_interfaces && m_slot_interfaces[i].name != NULL; i++)
 		if (!m_slot_interfaces[i].internal)
diff --git a/src/emu/dislot.h b/src/emu/dislot.h
index c1ad5a86144..bd5381718f2 100644
--- a/src/emu/dislot.h
+++ b/src/emu/dislot.h
@@ -59,7 +59,7 @@ public:
 	const void *default_config() const { return m_default_config; }
 	const UINT32 default_clock() const { return m_default_clock; }
 	const bool fixed() const { return m_fixed; }
-	const bool all_internal();
+	const bool all_internal() const;
 	device_t* get_card_device();
 protected:
 	const char *m_default_card;