diff --git a/build_tests/custom_builder/build_after.rb b/build_tests/custom_builder/build_after.rb
index e8c3e14..7d4697b 100644
--- a/build_tests/custom_builder/build_after.rb
+++ b/build_tests/custom_builder/build_after.rb
@@ -3,6 +3,8 @@ Rscons::Environment.new do |env|
               [],
               "CMD" => %w[ruby gen.rb ${_TARGET}],
               "CMD_DESC" => "Generating")
-  env.build_after("#{env.build_root}/program.o", "inc.c")
+  env["build_root"] = env.build_root
+  env["inc_c"] = "inc.c"
+  env.build_after("${build_root}/program.o", "${inc_c}")
   env.Program("program.exe", ["program.c", "inc.c"])
 end
diff --git a/lib/rscons/environment.rb b/lib/rscons/environment.rb
index 19fd825..f20b953 100644
--- a/lib/rscons/environment.rb
+++ b/lib/rscons/environment.rb
@@ -476,8 +476,10 @@ module Rscons
       targets = Array(targets)
       prerequisites = Array(prerequisites)
       targets.each do |target|
+        target = expand_path(expand_varref(target))
         @registered_build_dependencies[target] ||= Set.new
         prerequisites.each do |prerequisite|
+          prerequisite = expand_path(expand_varref(prerequisite))
           @registered_build_dependencies[target] << prerequisite
         end
       end