diff --git a/make/TestImage.gmk b/make/TestImage.gmk
index b719d6824da..f6d680fb690 100644
--- a/make/TestImage.gmk
+++ b/make/TestImage.gmk
@@ -28,6 +28,9 @@ default: all
 include $(SPEC)
 include MakeBase.gmk
 
+# Hook to include the corresponding custom file, if present.
+$(eval $(call IncludeCustomExtension, TestImage-pre.gmk))
+
 ############################################################################
 
 BUILD_INFO_PROPERTIES := $(TEST_IMAGE_DIR)/build-info.properties