diff --git a/packages/dune/virtualenv_from_envvariable.patch b/packages/dune/virtualenv_from_envvariable.patch
index 910ae22..2913527 100644
--- a/packages/dune/virtualenv_from_envvariable.patch
+++ b/packages/dune/virtualenv_from_envvariable.patch
@@ -6,7 +6,7 @@ index 6f8ff307..93ea3e9c 100644
    endif()
  endfunction()
  
-+if (IS_DIRECTORY "$ENV{DUNE_PYTHON_VIRTUALENV_PATH}")
++if (NOT "$ENV{DUNE_PYTHON_VIRTUALENV_PATH}" STREQUAL "")
 +  set(DUNE_PYTHON_VIRTUALENV_SETUP 1)
 +endif()
 +
@@ -21,7 +21,7 @@ index 8a09676f..96cb5954 100644
    "Place Python virtualenv in top-level directory \"dune-python-env\" when using an absolute build directory"
    )
  
-+if(IS_DIRECTORY "$ENV{DUNE_PYTHON_VIRTUALENV_PATH}")
++if (NOT "$ENV{DUNE_PYTHON_VIRTUALENV_PATH}" STREQUAL "")
 +  set(DUNE_PYTHON_VIRTUALENV_PATH "$ENV{DUNE_PYTHON_VIRTUALENV_PATH}")
 +endif()
 +