diff --git a/java/src/com/android/inputmethod/latin/BinaryDictionary.java b/java/src/com/android/inputmethod/latin/BinaryDictionary.java
index e084cb3..7184f1d 100644
--- a/java/src/com/android/inputmethod/latin/BinaryDictionary.java
+++ b/java/src/com/android/inputmethod/latin/BinaryDictionary.java
@@ -187,6 +187,9 @@
     }
 
     public static int editDistance(String before, String after) {
+        if (before == null || after == null) {
+            throw new IllegalArgumentException();
+        }
         return editDistanceNative(before.toCharArray(), after.toCharArray());
     }