diff --git a/imgutil/imgutil b/imgutil/imgutil
index d12886b5..5665c11d 100644
--- a/imgutil/imgutil
+++ b/imgutil/imgutil
@@ -243,7 +243,7 @@ def _mount_constrained_fs(opts, installroot):
         else:
             src = v
             dst = os.path.join(installroot, 'run/external')
-            while v and v[0] = '/':
+            while v and v[0] == '/':
                 v = v[1:]
             dst = os.path.join(dst, v)
         mkdirp(dst)