diff --git a/src/library/match.cpp b/src/library/match.cpp
index ad6023e7b..f1472b9c0 100644
--- a/src/library/match.cpp
+++ b/src/library/match.cpp
@@ -114,7 +114,7 @@ class match_fn : public match_context {
 
     optional<level> _get_subst(level const & x) const {
         unsigned i = to_meta_idx(x);
-        if (i > m_lsubst_sz)
+        if (i >= m_lsubst_sz)
             throw_exception();
         return m_lsubst[i];
     }