diff --git a/mfbt/MathAlgorithms.h b/mfbt/MathAlgorithms.h
index 927f008b213..ee36aca5082 100644
--- a/mfbt/MathAlgorithms.h
+++ b/mfbt/MathAlgorithms.h
@@ -414,13 +414,14 @@ FloorLog2Size(size_t n)
 }
 
 /*
- * Round x up to the nearest power of 2.  This function assumes that the most
- * significant bit of x is not set, which would lead to overflow.
+ * Compute the smallest power of 2 greater than or equal to |x|.  |x| must not
+ * be so great that the computed value would overflow |size_t|.
  */
 inline size_t
 RoundUpPow2(size_t x)
 {
-  MOZ_ASSERT(~x > x, "can't round up -- will overflow!");
+  MOZ_ASSERT(x <= (size_t(1) << (sizeof(size_t) * CHAR_BIT - 1)),
+             "can't round up -- will overflow!");
   return size_t(1) << CeilingLog2(x);
 }
 
diff --git a/mfbt/tests/TestCeilingFloor.cpp b/mfbt/tests/TestCeilingFloor.cpp
index fc697e230cf..8ac2fda4de9 100644
--- a/mfbt/tests/TestCeilingFloor.cpp
+++ b/mfbt/tests/TestCeilingFloor.cpp
@@ -7,6 +7,7 @@
 
 using mozilla::CeilingLog2;
 using mozilla::FloorLog2;
+using mozilla::RoundUpPow2;
 
 static void
 TestCeiling()
@@ -46,9 +47,39 @@ TestFloor()
     MOZ_ASSERT(FloorLog2(i) == 4);
 }
 
+static void
+TestRoundUpPow2()
+{
+  MOZ_ASSERT(RoundUpPow2(0) == 1);
+  MOZ_ASSERT(RoundUpPow2(1) == 1);
+  MOZ_ASSERT(RoundUpPow2(2) == 2);
+  MOZ_ASSERT(RoundUpPow2(3) == 4);
+  MOZ_ASSERT(RoundUpPow2(4) == 4);
+  MOZ_ASSERT(RoundUpPow2(5) == 8);
+  MOZ_ASSERT(RoundUpPow2(6) == 8);
+  MOZ_ASSERT(RoundUpPow2(7) == 8);
+  MOZ_ASSERT(RoundUpPow2(8) == 8);
+  MOZ_ASSERT(RoundUpPow2(9) == 16);
+
+  MOZ_ASSERT(RoundUpPow2(15) == 16);
+  MOZ_ASSERT(RoundUpPow2(16) == 16);
+  MOZ_ASSERT(RoundUpPow2(17) == 32);
+
+  MOZ_ASSERT(RoundUpPow2(31) == 32);
+  MOZ_ASSERT(RoundUpPow2(32) == 32);
+  MOZ_ASSERT(RoundUpPow2(33) == 64);
+
+  size_t MaxPow2 = size_t(1) << (sizeof(size_t) * CHAR_BIT - 1);
+  MOZ_ASSERT(RoundUpPow2(MaxPow2 - 1) == MaxPow2);
+  MOZ_ASSERT(RoundUpPow2(MaxPow2) == MaxPow2);
+  // not valid to round up when past the max power of two
+}
+
 int main()
 {
   TestCeiling();
   TestFloor();
+
+  TestRoundUpPow2();
   return 0;
 }