diff --git a/glm/detail/func_geometric.inl b/glm/detail/func_geometric.inl
index aec15efe..b3590152 100644
--- a/glm/detail/func_geometric.inl
+++ b/glm/detail/func_geometric.inl
@@ -114,9 +114,12 @@ namespace detail
 	{
 		GLM_FUNC_QUALIFIER static vec<L, T, Q> call(vec<L, T, Q> const& I, vec<L, T, Q> const& N, T eta)
 		{
+			assert(eta >= static_cast<T>(-1) && eta <= static_cast<T>(1));
+
 			T const dotValue(dot(N, I));
 			T const k(static_cast<T>(1) - eta * eta * (static_cast<T>(1) - dotValue * dotValue));
-			return (eta * I - (eta * dotValue + std::sqrt(k)) * N) * static_cast<T>(k >= static_cast<T>(0));
+			vec<L, T, Q> const Result = (eta * I - (eta * dotValue + std::sqrt(k)) * N) * static_cast<T>(k >= static_cast<T>(0));
+			return Result;
 		}
 	};
 }//namespace detail
diff --git a/glm/geometric.hpp b/glm/geometric.hpp
index e0380830..28d76199 100644
--- a/glm/geometric.hpp
+++ b/glm/geometric.hpp
@@ -99,6 +99,8 @@ namespace glm
 	/// and the ratio of indices of refraction eta,
 	/// return the refraction vector.
 	///
+	/// @param eta Indice of refraction. Must be a value between -1 and 1 inclusively.
+	///
 	/// @tparam L An integer between 1 and 4 included that qualify the dimension of the vector.
 	/// @tparam T Floating-point scalar types.
 	///
diff --git a/readme.md b/readme.md
index 16052c15..0670cd92 100644
--- a/readme.md
+++ b/readme.md
@@ -68,6 +68,7 @@ glm::mat4 camera(float Translate, glm::vec2 const& Rotate)
 - Splitted headers into EXT extensions to improve compilation time #670
 - Added separated performance tests
 - Refactor manual, lists all configuration defines
+- Clarified refract valid range of the indices of refraction, between -1 and 1 inclusively #806
 
 #### Fixes:
 - Fixed SIMD detection on Clang and GCC