diff --git a/lib/spack/spack/cmd/__init__.py b/lib/spack/spack/cmd/__init__.py
index d48301f5de..130abea4cc 100644
--- a/lib/spack/spack/cmd/__init__.py
+++ b/lib/spack/spack/cmd/__init__.py
@@ -435,7 +435,7 @@ def format_list(specs):
     out = ''
     if groups:
         for specs in iter_groups(specs, indent, all_headers):
-            out += format_list(specs)
+            output.write(format_list(specs))
     else:
         out = format_list(sorted(specs))