diff --git a/interpreter/cling/tools/plugins/clad/CMakeLists.txt b/interpreter/cling/tools/plugins/clad/CMakeLists.txt
index 48aea31eec7b2..3f5d1b275fad8 100644
--- a/interpreter/cling/tools/plugins/clad/CMakeLists.txt
+++ b/interpreter/cling/tools/plugins/clad/CMakeLists.txt
@@ -100,14 +100,17 @@ endif()
 # list(APPEND _clad_patches_list "patch1.patch" "patch2.patch")
 #list(APPEND _clad_patches_list "nullptr_dereference_fix.patch" "windows_build_fix.patch")
 list(APPEND _clad_patches_list "lld_lookup_no_default_path.patch")
-if (_clad_patches_list)
+# An external clad source directory is used as is: it is expected to be
+# already patched if necessary, and it might be read-only or not a git
+# repository, so the patches cannot be applied to it anyway.
+if (_clad_patches_list AND NOT DEFINED CLAD_SOURCE_DIR)
   set(_clad_patch_command
       ${CMAKE_COMMAND} -E copy_directory
       ${CMAKE_SOURCE_DIR}/interpreter/cling/tools/plugins/clad/patches <SOURCE_DIR>
       && git checkout -q <SOURCE_DIR>
       && git apply --ignore-space-change --ignore-whitespace ${_clad_patches_list}
       )
-endif(_clad_patches_list)
+endif(_clad_patches_list AND NOT DEFINED CLAD_SOURCE_DIR)
 
 ExternalProject_Add(
   clad