diff --git a/lib/rscons/builders/object.rb b/lib/rscons/builders/object.rb
index 6822021..b91916b 100644
--- a/lib/rscons/builders/object.rb
+++ b/lib/rscons/builders/object.rb
@@ -92,7 +92,7 @@ module Rscons
           '_DEPFILE' => Rscons.set_suffix(target, env.expand_varref("${DEPFILESUFFIX}", vars)),
         })
         com_prefix = KNOWN_SUFFIXES.find do |compiler, suffix_var|
-          sources.first.end_with?(*env.expand_varref("${#{suffix_var}}"))
+          sources.first.end_with?(*env.expand_varref("${#{suffix_var}}", vars))
         end.tap do |v|
           v.nil? and raise "Error: unknown input file type: #{sources.first.inspect}"
         end.first
diff --git a/lib/rscons/builders/shared_object.rb b/lib/rscons/builders/shared_object.rb
index 043ff4c..1f58d28 100644
--- a/lib/rscons/builders/shared_object.rb
+++ b/lib/rscons/builders/shared_object.rb
@@ -70,7 +70,7 @@ module Rscons
           '_DEPFILE' => Rscons.set_suffix(target, env.expand_varref("${DEPFILESUFFIX}", vars)),
         })
         com_prefix = KNOWN_SUFFIXES.find do |compiler, suffix_var|
-          sources.first.end_with?(*env.expand_varref("${#{suffix_var}}"))
+          sources.first.end_with?(*env.expand_varref("${#{suffix_var}}", vars))
         end.tap do |v|
           v.nil? and raise "Error: unknown input file type: #{sources.first.inspect}"
         end.first