diff --git a/sherpa-onnx/csrc/hypothesis.h b/sherpa-onnx/csrc/hypothesis.h
index 29a64340..659ef6d3 100644
--- a/sherpa-onnx/csrc/hypothesis.h
+++ b/sherpa-onnx/csrc/hypothesis.h
@@ -60,9 +60,9 @@ struct Hypothesis {
   std::string Key() const {
     // TODO(fangjun): Use a hash function?
     std::ostringstream os;
-    std::string sep = "-";
+    std::string sep;
     for (auto i : ys) {
-      os << i << sep;
+      os << sep << i;
       sep = "-";
     }
     return os.str();