diff --git a/transliteration.inc b/transliteration.inc
index 47659be..714706e 100644
--- a/transliteration.inc
+++ b/transliteration.inc
@@ -25,6 +25,10 @@
  *   Transliterated text.
  */
 function _transliteration_process($string, $unknown = '?', $source_langcode = NULL) {
+  if (!is_string($string)) {
+    return;
+  }
+
   // ASCII is always valid NFC! If we're only ever given plain ASCII, we can
   // avoid the overhead of initializing the decomposition tables by skipping
   // out early.