--- a/ports/cmake/cmake/read_api_version.cmake
+++ b/ports/cmake/cmake/read_api_version.cmake
@@ -1,6 +1,6 @@
 function(read_api_version project_version api_version outapi_version synapi_version )
 
-    file( READ "${CMAKE_SOURCE_DIR}/../../configure.ac" configure_ac )
+    file( READ "${CMAKE_CURRENT_SOURCE_DIR}/../../configure.ac" configure_ac )
 
     string( REGEX MATCH "AC_INIT\\(\\[mpg123\\], \\[([0-9\\.]+)" result ${configure_ac} )
     set( ${project_version} ${CMAKE_MATCH_1} PARENT_SCOPE )