diff --git a/lib/rscons/environment.rb b/lib/rscons/environment.rb
index 8a4ee5d..58b68b0 100644
--- a/lib/rscons/environment.rb
+++ b/lib/rscons/environment.rb
@@ -317,7 +317,7 @@ module Rscons
           end
 
           # If needed, do a blocking wait.
-          if job.nil? or @threaded_commands.size >= Rscons.n_threads
+          if (completed_tcs.empty? and job.nil?) or @threaded_commands.size >= Rscons.n_threads
             completed_tcs << wait_for_threaded_commands
           end