diff --git a/CMakeLists.txt b/CMakeLists.txt
index 082f94a..9c42a93 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -17,8 +17,11 @@ include(GroupSourceByFilesystem)
 include(ModernCpp)
 set_property(GLOBAL PROPERTY USE_FOLDERS ON)
 
-if (EXISTS "externals/udplibrary/CMakeLists.txt")
+if (EXISTS "${PROJECT_SOURCE_DIR}/externals/udplibrary/CMakeLists.txt")
+    message(STATUS "Detected udplibrary")
     set(HAVE_UDPLIBRARY 1)
+else()
+    message(FATAL_ERROR "udplibrary required... copy from swg source to the externals directory")
 endif()
 
 add_definitions(-DBOOST_ALL_NO_LIB)