diff --git a/include/CMakeLists.txt b/include/CMakeLists.txt
index 02f924d..62c0f62 100644
--- a/include/CMakeLists.txt
+++ b/include/CMakeLists.txt
@@ -15,7 +15,7 @@
 
 endif(INSTALL_MBEDTLS_HEADERS)
 
-# Make config.h available in an out-of-source build.
+# Make config.h available in an out-of-source build. ssl-opt.sh requires it.
 if (ENABLE_TESTING AND NOT ${CMAKE_CURRENT_BINARY_DIR} STREQUAL ${CMAKE_CURRENT_SOURCE_DIR})
     link_to_source(mbedtls)
     link_to_source(psa)