diff --git a/src/applications/conduit/parametertype/ConduitListParameterType.php b/src/applications/conduit/parametertype/ConduitListParameterType.php
--- a/src/applications/conduit/parametertype/ConduitListParameterType.php
+++ b/src/applications/conduit/parametertype/ConduitListParameterType.php
@@ -3,6 +3,17 @@
 abstract class ConduitListParameterType
   extends ConduitParameterType {
 
+  private $allowEmptyList = true;
+
+  public function setAllowEmptyList($allow_empty_list) {
+    $this->allowEmptyList = $allow_empty_list;
+    return $this;
+  }
+
+  public function getAllowEmptyList() {
+    return $this->allowEmptyList;
+  }
+
   protected function getParameterValue(array $request, $key) {
     $value = parent::getParameterValue($request, $key);
 
@@ -27,6 +38,13 @@
         pht('Expected a list, but value is an object.'));
     }
 
+    if (!$value && !$this->getAllowEmptyList()) {
+      $this->raiseValidationException(
+        $request,
+        $key,
+        pht('Expected a nonempty list, but value is an empty list.'));
+    }
+
     return $value;
   }
 
diff --git a/src/applications/conduit/parametertype/ConduitParameterType.php b/src/applications/conduit/parametertype/ConduitParameterType.php
--- a/src/applications/conduit/parametertype/ConduitParameterType.php
+++ b/src/applications/conduit/parametertype/ConduitParameterType.php
@@ -61,7 +61,11 @@
   protected function raiseValidationException(array $request, $key, $message) {
     // TODO: Specialize this so we can give users more tailored messages from
     // Conduit.
-    throw new Exception($message);
+    throw new Exception(
+      pht(
+        'Error while reading "%s": %s',
+        $key,
+        $message));
   }
 
 
diff --git a/src/applications/search/field/PhabricatorIDsSearchField.php b/src/applications/search/field/PhabricatorIDsSearchField.php
--- a/src/applications/search/field/PhabricatorIDsSearchField.php
+++ b/src/applications/search/field/PhabricatorIDsSearchField.php
@@ -24,7 +24,8 @@
   }
 
   protected function newConduitParameterType() {
-    return new ConduitIntListParameterType();
+    return id(new ConduitIntListParameterType())
+      ->setAllowEmptyList(false);
   }
 
 }
diff --git a/src/applications/search/field/PhabricatorPHIDsSearchField.php b/src/applications/search/field/PhabricatorPHIDsSearchField.php
--- a/src/applications/search/field/PhabricatorPHIDsSearchField.php
+++ b/src/applications/search/field/PhabricatorPHIDsSearchField.php
@@ -24,7 +24,8 @@
   }
 
   protected function newConduitParameterType() {
-    return new ConduitPHIDListParameterType();
+    return id(new ConduitPHIDListParameterType())
+      ->setAllowEmptyList(false);
   }
 
 }
diff --git a/src/applications/search/field/PhabricatorSearchDatasourceField.php b/src/applications/search/field/PhabricatorSearchDatasourceField.php
--- a/src/applications/search/field/PhabricatorSearchDatasourceField.php
+++ b/src/applications/search/field/PhabricatorSearchDatasourceField.php
@@ -22,7 +22,8 @@
 
   protected function newConduitParameterType() {
     if (!$this->conduitParameterType) {
-      return new ConduitStringListParameterType();
+      return id(new ConduitStringListParameterType())
+        ->setAllowEmptyList(false);
     }
 
     return $this->conduitParameterType;