--- a/pxr/external/boost/python/CMakeLists.txt
+++ b/pxr/external/boost/python/CMakeLists.txt
@@ -16,8 +16,8 @@ endif()
 
 foreach(config RELEASE RELWITHDEBINFO MINSIZEREL)
     string(REPLACE
-        ${ndebug_flag} ""
-        CMAKE_CXX_FLAGS_${config} ${CMAKE_CXX_FLAGS_${config}}
+        "${ndebug_flag}" ""
+        "CMAKE_CXX_FLAGS_${config}" "${CMAKE_CXX_FLAGS_${config}}"
     )
 endforeach()