update

src/Grid/Column/HasDisplayers.php

@@ -219,7 +219,7 @@ trait HasDisplayers
      *
      * @return $this
      */
-    public function valueAsFilter($valueKey = '', $operator = '=')
+    public function valueAsFilter($valueKey = null, $operator = '=')
     {
         $valueFilter = $this->getValueFilter();
 

src/Grid/Column/ValueFilter.php

@@ -131,6 +131,6 @@ class ValueFilter
             $pageName          => null,
         ]);
 
-        return "<a class='value-filter' href='{$url}'>{$this->wrap($value)}</a> &nbsp;<a style='opacity:0;' class='fa fa-filter'></a>";
+        return "<a class='value-filter' href='{$url}'>{$this->wrap($value)}</a> &nbsp;<a style='opacity:0;' class='fa fa-search'></a>";
     }
 }