diff --git a/var/spack/repos/builtin/packages/siesta/package.py b/var/spack/repos/builtin/packages/siesta/package.py
index 2779e8420a1..4060d90eb7c 100644
--- a/var/spack/repos/builtin/packages/siesta/package.py
+++ b/var/spack/repos/builtin/packages/siesta/package.py
@@ -28,6 +28,11 @@ class Siesta(Package):
 
     phases = ['configure', 'build', 'install']
 
+    def flag_handler(self, name, flags):
+        if '%gcc@10:' in self.spec and name == 'fflags':
+            flags.append('-fallow-argument-mismatch')
+        return (flags, None, None)
+
     def configure(self, spec, prefix):
         sh = which('sh')
         configure_args = ['--enable-mpi',