diff --git a/src/st/st-widget.c b/src/st/st-widget.c
index de459e3af..6c6fd3a3f 100644
--- a/src/st/st-widget.c
+++ b/src/st/st-widget.c
@@ -1199,6 +1199,7 @@ st_widget_add_style_class_name (StWidget    *actor,
 
   g_return_if_fail (ST_IS_WIDGET (actor));
   g_return_if_fail (style_class != NULL);
+  g_return_if_fail (style_class[0] != '\0');
 
   priv = st_widget_get_instance_private (actor);
 
@@ -1225,6 +1226,7 @@ st_widget_remove_style_class_name (StWidget    *actor,
 
   g_return_if_fail (ST_IS_WIDGET (actor));
   g_return_if_fail (style_class != NULL);
+  g_return_if_fail (style_class[0] != '\0');
 
   priv = st_widget_get_instance_private (actor);
 
@@ -1269,6 +1271,8 @@ st_widget_has_style_class_name (StWidget    *actor,
   StWidgetPrivate *priv;
 
   g_return_val_if_fail (ST_IS_WIDGET (actor), FALSE);
+  g_return_val_if_fail (style_class != NULL, FALSE);
+  g_return_val_if_fail (style_class[0] != '\0', FALSE);
 
   priv = st_widget_get_instance_private (actor);
 
@@ -1313,6 +1317,8 @@ st_widget_has_style_pseudo_class (StWidget    *actor,
   StWidgetPrivate *priv;
 
   g_return_val_if_fail (ST_IS_WIDGET (actor), FALSE);
+  g_return_val_if_fail (pseudo_class != NULL, FALSE);
+  g_return_val_if_fail (pseudo_class[0] != '\0', FALSE);
 
   priv = st_widget_get_instance_private (actor);
 
@@ -1362,6 +1368,7 @@ st_widget_add_style_pseudo_class (StWidget    *actor,
 
   g_return_if_fail (ST_IS_WIDGET (actor));
   g_return_if_fail (pseudo_class != NULL);
+  g_return_if_fail (pseudo_class[0] != '\0');
 
   priv = st_widget_get_instance_private (actor);
 
@@ -1387,6 +1394,7 @@ st_widget_remove_style_pseudo_class (StWidget    *actor,
 
   g_return_if_fail (ST_IS_WIDGET (actor));
   g_return_if_fail (pseudo_class != NULL);
+  g_return_if_fail (pseudo_class[0] != '\0');
 
   priv = st_widget_get_instance_private (actor);