diff --git a/src/muz/spacer/spacer_expand_bnd_generalizer.cpp b/src/muz/spacer/spacer_expand_bnd_generalizer.cpp
index a3e88f8d537..4f80be70604 100644
--- a/src/muz/spacer/spacer_expand_bnd_generalizer.cpp
+++ b/src/muz/spacer/spacer_expand_bnd_generalizer.cpp
@@ -102,7 +102,7 @@ lemma_expand_bnd_generalizer::lemma_expand_bnd_generalizer(context &ctx)
     // remove duplicates
     std::sort(m_values.begin(), m_values.end());
     auto last = std::unique(m_values.begin(), m_values.end());
-    for (unsigned i = 0, sz = std::distance(last, m_values.end()); i < sz; ++i)
+    for (size_t i = 0, sz = std::distance(last, m_values.end()); i < sz; ++i)
         m_values.pop_back();
 }