diff --git a/libc/include/signal.h b/libc/include/signal.h
index 9d3badc..e2713db 100644
--- a/libc/include/signal.h
+++ b/libc/include/signal.h
@@ -58,34 +58,34 @@
 extern const char* const sys_signame[];
 
 static __inline__ int sigismember(sigset_t* set, int signum) {
-  if (set == NULL || signum < 1 || signum >= 8*sizeof(sigset_t)) {
+  int bit = signum - 1; // Signal numbers start at 1, but bit positions start at 0.
+  if (set == NULL || bit < 0 || bit >= 8*sizeof(sigset_t)) {
     errno = EINVAL;
     return -1;
   }
   unsigned long* local_set = (unsigned long*) set;
-  signum--;
-  return (int) ((local_set[signum/LONG_BIT] >> (signum%LONG_BIT)) & 1);
+  return (int) ((local_set[bit / LONG_BIT] >> (bit % LONG_BIT)) & 1);
 }
 
 static __inline__ int sigaddset(sigset_t* set, int signum) {
-  if (set == NULL || signum < 1 || signum >= 8*sizeof(sigset_t)) {
+  int bit = signum - 1; // Signal numbers start at 1, but bit positions start at 0.
+  if (set == NULL || bit < 0 || bit >= 8*sizeof(sigset_t)) {
     errno = EINVAL;
     return -1;
   }
   unsigned long* local_set = (unsigned long*) set;
-  signum--;
-  local_set[signum/LONG_BIT] |= 1UL << (signum%LONG_BIT);
+  local_set[bit / LONG_BIT] |= 1UL << (bit % LONG_BIT);
   return 0;
 }
 
 static __inline__ int sigdelset(sigset_t* set, int signum) {
-  if (set == NULL || signum < 1 || signum >= 8*sizeof(sigset_t)) {
+  int bit = signum - 1; // Signal numbers start at 1, but bit positions start at 0.
+  if (set == NULL || bit < 0 || bit >= 8*sizeof(sigset_t)) {
     errno = EINVAL;
     return -1;
   }
   unsigned long* local_set = (unsigned long*) set;
-  signum--;
-  local_set[signum/LONG_BIT] &= ~(1UL << (signum%LONG_BIT));
+  local_set[bit / LONG_BIT] &= ~(1UL << (bit % LONG_BIT));
   return 0;
 }
 
diff --git a/tests/signal_test.cpp b/tests/signal_test.cpp
index fcfcb18..1292568 100644
--- a/tests/signal_test.cpp
+++ b/tests/signal_test.cpp
@@ -48,13 +48,13 @@
   int min_signal = SIGHUP;
   int max_signal = SIGRTMAX;
 
-#if __BIONIC__
-  // bionic's sigset_t is too small: 32 bits instead of 64.
+#if defined(__BIONIC__) && !defined(__mips__)
+  // bionic's sigset_t is too small for ARM and x86: 32 bits instead of 64.
   // This means you can't refer to any of the real-time signals.
   // See http://b/3038348 and http://b/5828899.
-  max_signal = 31;
+  max_signal = 32;
 #else
-  // Other C libraries are perfectly capable of using their largest signal.
+  // Other C libraries (or bionic for MIPS) are perfectly capable of using their largest signal.
   ASSERT_GE(sizeof(sigset_t) * 8, static_cast<size_t>(SIGRTMAX));
 #endif