diff --git a/include/gsl/span b/include/gsl/span
index 2fb5bbd..7c5b24d 100644
--- a/include/gsl/span
+++ b/include/gsl/span
@@ -25,7 +25,7 @@
 #include <array>     // for array
 #include <cstddef>   // for ptrdiff_t, size_t, nullptr_t
 #include <iterator>  // for reverse_iterator, distance, random_access_...
-#include <memory> // for std::addressof
+#include <memory>    // for std::addressof
 #include <stdexcept>
 #include <type_traits> // for enable_if_t, declval, is_convertible, inte...
 #include <utility>
@@ -475,7 +475,7 @@ public:
         typename details::calculate_subspan_type<ElementType, Extent, Offset, Count>::type
     {
         Expects((size() >= Offset) &&
-                (Count == dynamic_extent || (Offset + Count <= size())));
+                (Count == dynamic_extent || (Count <= size() - Offset)));
 
         return {data() + Offset, Count == dynamic_extent ? size() - Offset : Count};
     }
diff --git a/include/gsl/string_span b/include/gsl/string_span
index a1d2bb2..fc0dfeb 100644
--- a/include/gsl/string_span
+++ b/include/gsl/string_span
@@ -26,7 +26,7 @@
 #include <cstddef>   // for size_t, nullptr_t
 #include <cstdint>   // for PTRDIFF_MAX
 #include <cstring>
-#include <string>      // for basic_string, allocator, char_traits
+#include <string>    // for basic_string, allocator, char_traits
 #include <type_traits> // for declval, is_convertible, enable_if_t, add_...
 
 #if defined(_MSC_VER) && !defined(__clang__)