diff --git a/cruft/util/colour.cpp b/cruft/util/colour.cpp
index 22d4df07..bd3fd5c4 100644
--- a/cruft/util/colour.cpp
+++ b/cruft/util/colour.cpp
@@ -86,7 +86,7 @@ exponentiate (float const u)
 static cruft::vector3f
 sRGB_to_XYZ (cruft::vector3f sRGB)
 {
-    auto const [r_, g_, b_] = sRGB;
+    auto const &[r_, g_, b_] = sRGB;
 
     float const r = linearise (r_);
     float const g = linearise (g_);