diff --git a/src/base/span.h b/src/base/span.h
index 37b9b33d..f2e4e389 100644
--- a/src/base/span.h
+++ b/src/base/span.h
@@ -22,7 +22,7 @@ constexpr int Span_itemIndex(Span<T> span, typename Span<T>::const_reference ite
     auto index = &item - &span.front();
     assert(index >= 0);
     assert(index <= INT_MAX);
-    assert(&span[static_cast<Span<T>::size_type>(index)] == &item);
+    assert(&span[static_cast<typename Span<T>::size_type>(index)] == &item);
     return static_cast<int>(index);
 }