--- a/match/src/FullDTW.h
+++ b/match/src/FullDTW.h
@@ -83,7 +83,7 @@
      * against the best-matching subsequence of s1; otherwise it is
      * against the whole of s1.
      */
-    std::vector<size_t> align(const featureseq_t &s1,
+    std::vector<std::size_t> align(const featureseq_t &s1,
                               const featureseq_t &s2);
 
 private: