diff --git a/src/Form/Filters/Constraints/ParameterConstraintType.php b/src/Form/Filters/Constraints/ParameterConstraintType.php
index 52fbc726..3c3b396d 100644
--- a/src/Form/Filters/Constraints/ParameterConstraintType.php
+++ b/src/Form/Filters/Constraints/ParameterConstraintType.php
@@ -50,10 +50,12 @@ class ParameterConstraintType extends AbstractType
 
         $builder->add('unit', SearchType::class, [
             'required' => false,
+            'empty_data' => '',
         ]);
 
         $builder->add('symbol', SearchType::class, [
-            'required' => false
+            'required' => false,
+            'empty_data' => '',
         ]);
 
         $builder->add('value_text', TextConstraintType::class, [