diff --git a/tests/signal_test.cpp b/tests/signal_test.cpp
index 36ac690..207c156 100644
--- a/tests/signal_test.cpp
+++ b/tests/signal_test.cpp
@@ -527,3 +527,11 @@
   ASSERT_EQ(0, sigprocmask(SIG_BLOCK, nullptr, &set));
   EXPECT_TRUE(sigismember(&set, SIGALRM));
 }
+
+TEST(signal, killpg_EINVAL) {
+  // POSIX leaves pgrp <= 1 undefined, but glibc fails with EINVAL for < 0
+  // and passes 0 through to kill(2).
+  errno = 0;
+  ASSERT_EQ(-1, killpg(-1, SIGKILL));
+  ASSERT_EQ(EINVAL, errno);
+}