diff --git a/include/gsl/span b/include/gsl/span
index 5900da5..c480f18 100644
--- a/include/gsl/span
+++ b/include/gsl/span
@@ -182,7 +182,7 @@ namespace details
         {
             Expects(begin_ && current_ && end_);
             if (n > 0) Expects(end_ - current_ >= n);
-            if (n < 0) Expects(end_ - current_ >= -n);
+            if (n < 0) Expects(current_ - begin_ >= -n);
             current_ += n;
             return *this;
         }