diff --git a/image_properties.h b/image_properties.h
index 8f8c0f6..6026c2e 100644
--- a/image_properties.h
+++ b/image_properties.h
@@ -74,7 +74,7 @@
                                  const MutableImageProperties& properties);
 
 // Sets the root_prefix used to load files from during unittests to
-// |test_root_prefix|.
+// |test_root_prefix|. Passing a nullptr value resets it to the default.
 namespace test {
 void SetImagePropertiesRootPrefix(const char* test_root_prefix);
 }  // namespace test