diff --git a/var/spack/repos/builtin/packages/sloccount/package.py b/var/spack/repos/builtin/packages/sloccount/package.py
index 773d40acb9e..ea642ab669b 100644
--- a/var/spack/repos/builtin/packages/sloccount/package.py
+++ b/var/spack/repos/builtin/packages/sloccount/package.py
@@ -19,6 +19,7 @@ class Sloccount(MakefilePackage):
 
     # md5sum needed at run-time
     depends_on('coreutils', type=('build', 'run'))
+    depends_on('flex', type='build')
 
     def edit(self, spec, prefix):
         makefile = FileFilter('makefile')