diff --git a/externals/Makefile.am b/externals/Makefile.am
index 937e095a31..0c4c1921cd 100644
--- a/externals/Makefile.am
+++ b/externals/Makefile.am
@@ -37,7 +37,7 @@ endif
 
 ATERM = aterm-2.4.2-fixes
 
-$(ATERM).tar.gz:
+$(ATERM).tar.bz2:
 	@echo "Nix requires the CWI ATerm library to build."
 	@echo "Please download version 2.4.2-fixes from"
 	@echo "  http://losser.st-lab.cs.uu.nl/~eelco/dist/aterm-2.4.2-fixes.tar.bz2"