diff -Naur xfce4-panel-4.13.5/plugins/tasklist/tasklist-widget.c xfce4-panel-4.13.5-slack-icon-mssing-bug/plugins/tasklist/tasklist-widget.c
--- xfce4-panel-4.13.5/plugins/tasklist/tasklist-widget.c	2019-04-20 17:42:51.000000000 -0400
+++ xfce4-panel-4.13.5-slack-icon-mssing-bug/plugins/tasklist/tasklist-widget.c	2019-05-31 09:47:16.391164845 -0400
@@ -616,7 +616,32 @@
   gtk_widget_show (tasklist->arrow_button);
 }
 
-
+static GdkPixbuf *
+xfce_tasklist_get_icon(WnckWindow *window, gboolean show_labels, int size, int type)
+{
+  GdkPixbuf *pixbuf;
+  if (show_labels || type == CHILD_TYPE_GROUP_MENU)
+    {
+      pixbuf = wnck_window_get_mini_icon (window);
+    }
+  else if (size <= 31)
+    {
+      pixbuf = wnck_window_get_mini_icon (window);
+    }
+  else
+    {	  
+      pixbuf = wnck_window_get_icon (window);
+    }
+  if (!pixbuf || wnck_window_get_icon_is_fallback (window))
+    {
+      GtkIconTheme *theme = gtk_icon_theme_get_default ();
+      const char   *name = wnck_window_get_class_instance_name (window);
+      // Get the pixbuf from the icon theme
+      pixbuf = gtk_icon_theme_load_icon (theme, name, size,
+	  GTK_ICON_LOOKUP_FORCE_SIZE, NULL);
+    }
+  return pixbuf;
+}
 
 static void
 xfce_tasklist_get_property (GObject    *object,
@@ -2553,14 +2578,7 @@
   context = gtk_widget_get_style_context (GTK_WIDGET (child->icon));
 
   /* get the window icon */
-  if (tasklist->show_labels ||
-      child->type == CHILD_TYPE_GROUP_MENU)
-    pixbuf = wnck_window_get_mini_icon (window);
-  else if (icon_size <= 31)
-    pixbuf = wnck_window_get_mini_icon (window);
-  else
-    pixbuf = wnck_window_get_icon (window);
-
+  pixbuf = xfce_tasklist_get_icon(child->window, tasklist->show_labels, icon_size, child->type);
   /* leave when there is no valid pixbuf */
   if (G_UNLIKELY (pixbuf == NULL))
     {
@@ -3259,7 +3277,7 @@
     }
 #endif
 
-  pixbuf = wnck_window_get_icon (child->window);
+  pixbuf = xfce_tasklist_get_icon (child->window, FALSE, 32, CHILD_TYPE_WINDOW);
   if (G_LIKELY (pixbuf != NULL))
     gtk_drag_set_icon_pixbuf (context, pixbuf, 0, 0);
 }
@@ -3318,8 +3336,6 @@
     }
 }
 
-
-
 static XfceTasklistChild *
 xfce_tasklist_button_new (WnckWindow   *window,
                           XfceTasklist *tasklist)
@@ -3701,8 +3717,6 @@
     xfce_tasklist_sort (group_child->tasklist);
 }
 
-
-
 static void
 xfce_tasklist_group_button_icon_changed (WnckClassGroup    *class_group,
                                          XfceTasklistChild *group_child)