diff --git a/PyPreferences.jl/src/which.jl b/PyPreferences.jl/src/which.jl
index 94539d3c..ff2a2b7a 100644
--- a/PyPreferences.jl/src/which.jl
+++ b/PyPreferences.jl/src/which.jl
@@ -1,3 +1,7 @@
+# This file vendors the utility `Sys.which` for Julia versions prior to 1.7,
+# as in 1.6 it called `realpath` and therefore breaks virtual environment
+# detection. (Without this, all `test/test_venv.jl` would fail).
+
 @static if VERSION >= v"1.7.0"
     const _which = Sys.which
 else