--- a/mfbt/MathAlgorithms.h
+++ b/mfbt/MathAlgorithms.h
@@ -165,26 +165,28 @@ namespace mozilla {
 namespace detail {
 
 #if defined(MOZ_BITSCAN_WINDOWS)
 
 inline uint_fast8_t
 CountLeadingZeroes32(uint32_t aValue)
 {
   unsigned long index;
-  _BitScanReverse(&index, static_cast<unsigned long>(aValue));
+  if (!_BitScanReverse(&index, static_cast<unsigned long>(aValue)))
+      return 32;
   return uint_fast8_t(31 - index);
 }
 
 
 inline uint_fast8_t
 CountTrailingZeroes32(uint32_t aValue)
 {
   unsigned long index;
-  _BitScanForward(&index, static_cast<unsigned long>(aValue));
+  if (!_BitScanForward(&index, static_cast<unsigned long>(aValue)))
+      return 32;
   return uint_fast8_t(index);
 }
 
 inline uint_fast8_t
 CountPopulation32(uint32_t aValue)
 {
   uint32_t x = aValue - ((aValue >> 1) & 0x55555555);
   x = (x & 0x33333333) + ((x >> 2) & 0x33333333);
@@ -197,33 +199,35 @@ CountPopulation64(uint64_t aValue)
                       CountPopulation32(aValue >> 32));
 }
 
 inline uint_fast8_t
 CountLeadingZeroes64(uint64_t aValue)
 {
 #if defined(MOZ_BITSCAN_WINDOWS64)
   unsigned long index;
-  _BitScanReverse64(&index, static_cast<unsigned __int64>(aValue));
+  if (!_BitScanReverse64(&index, static_cast<unsigned __int64>(aValue)))
+      return 64;
   return uint_fast8_t(63 - index);
 #else
   uint32_t hi = uint32_t(aValue >> 32);
   if (hi != 0) {
     return CountLeadingZeroes32(hi);
   }
   return 32u + CountLeadingZeroes32(uint32_t(aValue));
 #endif
 }
 
 inline uint_fast8_t
 CountTrailingZeroes64(uint64_t aValue)
 {
 #if defined(MOZ_BITSCAN_WINDOWS64)
   unsigned long index;
-  _BitScanForward64(&index, static_cast<unsigned __int64>(aValue));
+  if (!_BitScanForward64(&index, static_cast<unsigned __int64>(aValue)))
+      return 64;
   return uint_fast8_t(index);
 #else
   uint32_t lo = uint32_t(aValue);
   if (lo != 0) {
     return CountTrailingZeroes32(lo);
   }
   return 32u + CountTrailingZeroes32(uint32_t(aValue >> 32));
 #endif