diff --git a/gvc-mixer-control.c b/gvc-mixer-control.c
index de1c574..f16829d 100644
--- a/gvc-mixer-control.c
+++ b/gvc-mixer-control.c
@@ -235,6 +235,9 @@ gvc_mixer_control_lookup_device_from_stream (GvcMixerControl *control,
         const GList             *ports;
         GvcMixerUIDevice        *ret;
 
+        g_return_val_if_fail (GVC_IS_MIXER_CONTROL (control), NULL);
+        g_return_val_if_fail (GVC_IS_MIXER_STREAM (stream), NULL);
+
         if (GVC_IS_MIXER_SOURCE (stream))
                devices = g_hash_table_get_values (control->priv->ui_inputs);
         else
@@ -534,6 +537,9 @@ gvc_mixer_control_change_profile_on_selected_device (GvcMixerControl  *control,
         GvcMixerCardProfile *current_profile;
         GvcMixerCard        *card;
 
+        g_return_val_if_fail (GVC_IS_MIXER_CONTROL (control), FALSE);
+        g_return_val_if_fail (GVC_IS_MIXER_UI_DEVICE (device), FALSE);
+
         g_object_get (G_OBJECT (device), "card", &card, NULL);
         current_profile = gvc_mixer_card_get_profile (card);
 
@@ -588,6 +594,9 @@ gvc_mixer_control_change_output (GvcMixerControl *control,
         const GvcMixerStreamPort *active_port;
         const gchar              *output_port;
 
+        g_return_if_fail (GVC_IS_MIXER_CONTROL (control));
+        g_return_if_fail (GVC_IS_MIXER_UI_DEVICE (output));
+
         g_debug ("control change output");
 
         stream = gvc_mixer_control_get_stream_from_device (control, output);
@@ -677,6 +686,9 @@ gvc_mixer_control_change_input (GvcMixerControl *control,
         const GvcMixerStreamPort *active_port;
         const gchar              *input_port;
 
+        g_return_if_fail (GVC_IS_MIXER_CONTROL (control));
+        g_return_if_fail (GVC_IS_MIXER_UI_DEVICE (input));
+
         stream = gvc_mixer_control_get_stream_from_device (control, input);
         if (stream == NULL) {
                 gvc_mixer_control_change_profile_on_selected_device (control,
@@ -908,7 +920,7 @@ dec_outstanding (GvcMixerControl *control)
 GvcMixerControlState
 gvc_mixer_control_get_state (GvcMixerControl *control)
 {
-        g_return_val_if_fail (GVC_IS_MIXER_CONTROL (control), FALSE);
+        g_return_val_if_fail (GVC_IS_MIXER_CONTROL (control), GVC_STATE_CLOSED);
 
         return control->priv->state;
 }
@@ -2184,6 +2196,8 @@ gvc_mixer_control_set_headset_port (GvcMixerControl      *control,
                                     guint                 id,
                                     GvcHeadsetPortChoice  choice)
 {
+        g_return_if_fail (GVC_IS_MIXER_CONTROL (control));
+
 #ifdef HAVE_ALSA
         switch (choice) {
         case GVC_HEADSET_PORT_CHOICE_HEADPHONES:
@@ -3730,11 +3744,13 @@ gvc_mixer_control_new (const char *name)
 gdouble
 gvc_mixer_control_get_vol_max_norm (GvcMixerControl *control)
 {
+	g_return_val_if_fail (GVC_IS_MIXER_CONTROL (control), 0);
 	return (gdouble) PA_VOLUME_NORM;
 }
 
 gdouble
 gvc_mixer_control_get_vol_max_amplified (GvcMixerControl *control)
 {
+	g_return_val_if_fail (GVC_IS_MIXER_CONTROL (control), 0);
 	return (gdouble) PA_VOLUME_UI_MAX;
 }
diff --git a/gvc-mixer-stream.c b/gvc-mixer-stream.c
index b136a14..af13ba3 100644
--- a/gvc-mixer-stream.c
+++ b/gvc-mixer-stream.c
@@ -839,6 +839,8 @@ gvc_mixer_stream_change_is_muted (GvcMixerStream *stream,
 gboolean
 gvc_mixer_stream_is_running (GvcMixerStream *stream)
 {
+        g_return_val_if_fail (GVC_IS_MIXER_STREAM (stream), FALSE);
+
         if (stream->priv->change_volume_op == NULL)
                 return FALSE;
 
diff --git a/gvc-mixer-ui-device.c b/gvc-mixer-ui-device.c
index 9b6addc..f7dd33e 100644
--- a/gvc-mixer-ui-device.c
+++ b/gvc-mixer-ui-device.c
@@ -339,6 +339,9 @@ gvc_mixer_ui_device_get_matching_profile (GvcMixerUIDevice *device, const gchar
         GList *l;
         gchar *result = NULL;
 
+        g_return_val_if_fail (GVC_IS_MIXER_UI_DEVICE (device), NULL);
+        g_return_val_if_fail (profile != NULL, NULL);
+
         for (l = device->priv->profiles; l != NULL; l = l->next) {
                 gchar *canonical_name;
                 GvcMixerCardProfile* p = l->data;
@@ -424,6 +427,8 @@ gvc_mixer_ui_device_set_profiles (GvcMixerUIDevice *device,
         GHashTable *added_profiles;
         const gchar *skip_prefix = device->priv->type == UIDeviceInput ? "output:" : "input:";
 
+        g_return_if_fail (GVC_IS_MIXER_UI_DEVICE (device));
+
         g_debug ("Set profiles for '%s'", gvc_mixer_ui_device_get_description(device));
 
         if (in_profiles == NULL)
@@ -462,6 +467,9 @@ gvc_mixer_ui_device_get_best_profile (GvcMixerUIDevice *device,
         const gchar *skip_prefix;
         gchar *canonical_name_selected;
 
+        g_return_val_if_fail (GVC_IS_MIXER_UI_DEVICE (device), NULL);
+        g_return_val_if_fail (current != NULL, NULL);
+
         if (device->priv->type == UIDeviceInput)
                 skip_prefix = "output:";
         else
@@ -657,6 +665,8 @@ gvc_mixer_ui_device_get_gicon (GvcMixerUIDevice *device)
 {
         const char *icon_name;
 
+        g_return_val_if_fail (GVC_IS_MIXER_UI_DEVICE (device), NULL);
+
         icon_name = gvc_mixer_ui_device_get_icon_name (device);
 
         if (icon_name != NULL)
@@ -700,6 +710,7 @@ gvc_mixer_ui_device_set_user_preferred_profile (GvcMixerUIDevice *device,
                                                 const gchar      *profile)
 {
         g_return_if_fail (GVC_IS_MIXER_UI_DEVICE (device));
+        g_return_if_fail (profile != NULL);
 
         g_free (device->priv->user_preferred_profile);
         device->priv->user_preferred_profile = g_strdup (profile);