diff --git a/include/gsl/span b/include/gsl/span
index 4f3b018..f5e850a 100644
--- a/include/gsl/span
+++ b/include/gsl/span
@@ -187,7 +187,7 @@ 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;
         }
@@ -294,7 +294,7 @@ namespace details
         }
 
         constexpr void _Verify_offset(const difference_type n) const noexcept
-        { // test that the iterator *this + n is a valid range in an STL
+        { // test that *this + n is within the span of this iterator STL
             // algorithm call
             Expects((current_ + n) >= begin_ && (current_ + n) <= end_);
         }