Error message

Notice: Uninitialized string offset: -1 in Drupal\bibcite_ris\Encoder\RISEncoder->buildEntry() (line 220 of modules/contrib/bibcite/modules/bibcite_ris/src/Encoder/RISEncoder.php).
Drupal\bibcite_ris\Encoder\RISEncoder->buildEntry(Array) (Line: 185)
Drupal\bibcite_ris\Encoder\RISEncoder->Drupal\bibcite_ris\Encoder\{closure}(Array)
array_map(Object, Array) (Line: 184)
Drupal\bibcite_ris\Encoder\RISEncoder->encode(Array, 'ris', Array) (Line: 40)
Symfony\Component\Serializer\Encoder\ChainEncoder->encode(Array, 'ris', Array) (Line: 306)
Symfony\Component\Serializer\Serializer->encode(Array, 'ris', Array) (Line: 129)
Symfony\Component\Serializer\Serializer->serialize(Array, 'ris') (Line: 75)
Drupal\bibcite_export\Controller\ExportController->processExport(Array, Object, 'bibcite_reference-798-RIS') (Line: 113)
Drupal\bibcite_export\Controller\ExportController->export(Object, 'bibcite_reference', Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 564)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 158)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 80)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 58)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 191)
Drupal\page_cache\StackMiddleware\PageCache->fetch(Object, 1, 1) (Line: 128)
Drupal\page_cache\StackMiddleware\PageCache->lookup(Object, 1, 1) (Line: 82)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 709)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)
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