diff --git a/runtime/vm/unicode.cc b/runtime/vm/unicode.cc
index a3b4fb9..90379d1 100644
--- a/runtime/vm/unicode.cc
+++ b/runtime/vm/unicode.cc
@@ -344,6 +344,7 @@
       return false;  // Invalid input.
     }
     if (is_supplementary) {
+      if (j == (len - 1)) return false;  // Output overflow.
       Utf16::Encode(ch, &dst[j]);
       j = j + 1;
     } else {