diff --git a/libc/calls/sig.c b/libc/calls/sig.c
index 86af41b35..eb1074dbc 100644
--- a/libc/calls/sig.c
+++ b/libc/calls/sig.c
@@ -29,6 +29,8 @@
 
 struct Signals __sig;  // TODO(jart): Need TLS
 
+#define GetSigBit(XXSIG) (1ull << (((XXSIG)-1) & 63))
+
 /**
  * Changes signal mask for main thread.
  * @return 0 on success, or -1 w/ errno
@@ -51,7 +53,7 @@ textwindows int __sig_mask(int how, const sigset_t *neu, sigset_t *old) {
           __sig.mask.__bits[i] = neu->__bits[i];
         }
       }
-      __sig.mask.__bits[0] &= ~(SIGKILL | SIGSTOP);
+      __sig.mask.__bits[0] &= ~(GetSigBit(SIGKILL) | GetSigBit(SIGSTOP));
     }
     __sig_unlock();
     return 0;
@@ -59,3 +61,5 @@ textwindows int __sig_mask(int how, const sigset_t *neu, sigset_t *old) {
     return einval();
   }
 }
+
+#undef GetSigBit
diff --git a/test/libc/calls/sigprocmask_test.c b/test/libc/calls/sigprocmask_test.c
index 1b155fbc5..aff4a62f0 100644
--- a/test/libc/calls/sigprocmask_test.c
+++ b/test/libc/calls/sigprocmask_test.c
@@ -78,3 +78,18 @@ TEST(sigprocmask, testMultipleBlockedDeliveriesOfSameSignal) {
     EXPECT_EQ(1, n);
   }
 }
+
+TEST(sigprocmask, testBlockingSIGINT) {
+  sigset_t neu, old;
+  struct sigaction oldsigint;
+  struct sigaction sa = {.sa_sigaction = OnSig, .sa_flags = SA_SIGINFO};
+  n = 0;
+  sigemptyset(&neu);
+  sigaddset(&neu, SIGINT);
+  EXPECT_EQ(0, sigprocmask(SIG_BLOCK, &neu, &old));
+  ASSERT_EQ(0, sigaction(SIGINT, &sa, &oldsigint));
+  ASSERT_EQ(0, raise(SIGINT));
+  EXPECT_EQ(0, n);
+  EXPECT_EQ(0, sigprocmask(SIG_SETMASK, &old, NULL));
+  EXPECT_EQ(0, sigaction(SIGINT, &oldsigint, 0));
+}