diff --git a/gsl/gsl_util b/gsl/gsl_util
index f0ac964..889414a 100644
--- a/gsl/gsl_util
+++ b/gsl/gsl_util
@@ -135,30 +135,30 @@ inline T narrow(U u)
 // at() - Bounds-checked way of accessing static arrays, std::array, std::vector
 //
 template <class T, size_t N>
-constexpr T& at(T (&arr)[N], size_t index)
+constexpr T& at(T (&arr)[N], std::ptrdiff_t index)
 {
-    Expects(index < N);
-    return arr[index];
+    Expects(index >= 0 && index < narrow_cast<std::ptrdiff_t>(N));
+    return arr[static_cast<size_t>(index)];
 }
 
 template <class T, size_t N>
-constexpr T& at(std::array<T, N>& arr, size_t index)
+constexpr T& at(std::array<T, N>& arr, std::ptrdiff_t index)
 {
-    Expects(index < N);
-    return arr[index];
+    Expects(index >= 0 && index < narrow_cast<std::ptrdiff_t>(N));
+    return arr[static_cast<size_t>(index)];
 }
 
 template <class Cont>
-constexpr typename Cont::value_type& at(Cont& cont, size_t index)
+constexpr typename Cont::value_type& at(Cont& cont, std::ptrdiff_t index)
 {
-    Expects(index < cont.size());
-    return cont[index];
+    Expects(index >= 0 && index < narrow_cast<std::ptrdiff_t>(cont.size()));
+    return cont[static_cast<typename Cont::size_type>(index)];
 }
 
 template <class T>
-constexpr const T& at(std::initializer_list<T> cont, size_t index)
+constexpr const T& at(std::initializer_list<T> cont, std::ptrdiff_t index)
 {
-    Expects(index < cont.size());
+    Expects(index >= 0 && index < narrow_cast<std::ptrdiff_t>(cont.size()));
     return *(cont.begin() + index);
 }
 
diff --git a/gsl/span b/gsl/span
index cf90e86..07c59c5 100644
--- a/gsl/span
+++ b/gsl/span
@@ -627,7 +627,7 @@ as_writeable_bytes(span<ElementType, Extent> s) noexcept
 
 // Specialization of gsl::at for span
 template <class ElementType, std::ptrdiff_t Extent>
-constexpr ElementType& at(const span<ElementType, Extent>& s, size_t index)
+constexpr ElementType& at(const span<ElementType, Extent>& s, std::ptrdiff_t index)
 {
     // No bounds checking here because it is done in span::operator[] called below
     return s[index];
diff --git a/tests/at_tests.cpp b/tests/at_tests.cpp
index 008fddf..166eb9b 100644
--- a/tests/at_tests.cpp
+++ b/tests/at_tests.cpp
@@ -31,6 +31,7 @@ SUITE(at_tests)
         for (int i = 0; i < 4; ++i)
             CHECK(at(a, i) == i+1);
 
+        CHECK_THROW(at(a, -1), fail_fast);
         CHECK_THROW(at(a, 4), fail_fast);
     }
 
@@ -41,6 +42,7 @@ SUITE(at_tests)
         for (int i = 0; i < 4; ++i)
             CHECK(at(a, i) == i+1);
 
+        CHECK_THROW(at(a, -1), fail_fast);
         CHECK_THROW(at(a, 4), fail_fast);
     }
 
@@ -51,6 +53,7 @@ SUITE(at_tests)
         for (int i = 0; i < 4; ++i)
             CHECK(at(a, i) == i+1);
 
+        CHECK_THROW(at(a, -1), fail_fast);
         CHECK_THROW(at(a, 4), fail_fast);
     }
 
@@ -61,6 +64,7 @@ SUITE(at_tests)
         for (int i = 0; i < 4; ++i)
             CHECK(at(a, i) == i+1);
 
+        CHECK_THROW(at(a, -1), fail_fast);
         CHECK_THROW(at(a, 4), fail_fast);
     }
 }