diff --git a/include/errors/class_SimplePluginCheckError.inc b/include/errors/class_SimplePluginCheckError.inc
index de9772773314618b41185152351d1992c5b9454a..c9679e4b3dfe05409c5af64e25119ea695be484c 100644
--- a/include/errors/class_SimplePluginCheckError.inc
+++ b/include/errors/class_SimplePluginCheckError.inc
@@ -83,4 +83,20 @@ class SimplePluginCheckError extends FusionDirectoryError
 
     return [_('Error'), $html, ERROR_DIALOG, $trace];
   }
+
+  /*!
+   * \brief Format error message for invalid value
+   *
+   * \param string $error The unescaped text error
+   *
+   * \return HTML string
+   */
+  static public function invalidValue(string $error): string
+  {
+    return sprintf(
+      '<b>%s</b> %s',
+      htmlescape(_('Invalid value:')),
+      htmlescape($error)
+    );
+  }
 }
diff --git a/include/simpleplugin/attributes/class_IntAttribute.inc b/include/simpleplugin/attributes/class_IntAttribute.inc
index bcbfc7008d0223d3640ea347ad8805f5d365b59a..b5218747a7434534ad7d851085c9da2cf8a43e87 100644
--- a/include/simpleplugin/attributes/class_IntAttribute.inc
+++ b/include/simpleplugin/attributes/class_IntAttribute.inc
@@ -81,13 +81,22 @@ class IntAttribute extends Attribute
       return $error;
     } elseif ($this->value !== '') {
       if (!is_numeric($this->value)) {
-        return new SimplePluginCheckError($this, sprintf(_('"%s" is not an number'), $this->getValue()));
+        return new SimplePluginCheckError(
+          $this,
+          SimplePluginCheckError::invalidValue(sprintf(_('"%s" is not an number'), $this->getValue()))
+        );
       }
       if (($this->min !== FALSE) && ($this->value < $this->min)) {
-        return new SimplePluginCheckError($this, sprintf(_('%s is smaller than %s'), $this->getValue(), $this->min));
+        return new SimplePluginCheckError(
+          $this,
+          SimplePluginCheckError::invalidValue(sprintf(_('%s is smaller than %s'), $this->getValue(), $this->min))
+        );
       }
       if (($this->max !== FALSE) && ($this->value > $this->max)) {
-        return new SimplePluginCheckError($this, sprintf(_('%s is larger than %s'), $this->getValue(), $this->max));
+        return new SimplePluginCheckError(
+          $this,
+          SimplePluginCheckError::invalidValue(sprintf(_('%s is larger than %s'), $this->getValue(), $this->max))
+        );
       }
     }
   }