diff --git a/var/spack/repos/builtin/packages/flex/package.py b/var/spack/repos/builtin/packages/flex/package.py
index 800e4b9d96a..9b173bb0dd6 100644
--- a/var/spack/repos/builtin/packages/flex/package.py
+++ b/var/spack/repos/builtin/packages/flex/package.py
@@ -35,6 +35,7 @@ class Flex(Package):
     version('2.5.39', 'e133e9ead8ec0a58d81166b461244fde')
 
     depends_on("bison", type='build')
+    depends_on("m4", type='build')
 
     def install(self, spec, prefix):
         configure("--prefix=%s" % prefix)