diff --git a/test/mkmf/test_flags.rb b/test/mkmf/test_flags.rb
index 6fde46adec..3d908c6bb4 100644
--- a/test/mkmf/test_flags.rb
+++ b/test/mkmf/test_flags.rb
@@ -33,7 +33,7 @@ class TestMkmfFlags < TestMkmf
   end
 
   def test_try_ldflag_invalid_opt
-    assert_separately([], [], <<-'end;') #do
+    assert_separately([], [], <<-'end;', timeout: 30) #do
       assert(!try_ldflags("nosuch.c"), TestMkmf::MKMFLOG)
       assert(have_devel?, TestMkmf::MKMFLOG)
     end;
@@ -47,7 +47,7 @@ class TestMkmfFlags < TestMkmf
   end
 
   def test_try_cppflag_invalid_opt
-    assert_separately([], [], <<-'end;') #do
+    assert_separately([], [], <<-'end;', timeout: 30) #do
       assert(!try_cppflags("nosuch.c"), TestMkmf::MKMFLOG)
       assert(have_devel?, TestMkmf::MKMFLOG)
     end;