diff --git a/mfbt/BinarySearch.h b/mfbt/BinarySearch.h
index d8782694e7f6..870be6e6af7c 100644
--- a/mfbt/BinarySearch.h
+++ b/mfbt/BinarySearch.h
@@ -6,6 +6,7 @@
 #define mozilla_BinarySearch_h
 
 #include <cstddef>
+#include <type_traits>
 #include <utility>
 
 #include "mozilla/Assertions.h"
@@ -32,8 +33,8 @@ namespace mozilla {
  *
  * The BinarySearchIf() version behaves similarly, but takes |aComparator|, a
  * functor to compare the values with, instead of a value to find.
- * That functor should take one argument - the value to compare - and return an
- * |int| with the comparison result:
+ * That functor should take one argument - the value to compare - and return a
+ * signed integer with the comparison result:
  *
  *   * 0, if the argument is equal to,
  *   * less than 0, if the argument is greater than,
@@ -57,7 +58,8 @@ namespace mozilla {
  *
  *   size_t match;
  *   if (BinarySearchIf(sortedInts, 0, sortedInts.length(), Comparator(13),
- * &match)) { printf("found 13 at %lu\n", match);
+ *                      &match)) {
+ *     printf("found 13 at %lu\n", match);
  *   }
  *
  */
@@ -76,6 +78,8 @@ bool BinarySearchIf(const Container& aContainer, size_t aBegin, size_t aEnd,
     // Allow any intermediate type so long as it provides a suitable ordering
     // relation.
     const auto result = aCompare(aContainer[middle]);
+    static_assert(!std::is_unsigned_v<decltype(result)>,
+                  "comparator result must allow negative values");
 
     if (result == 0) {
       *aMatchOrInsertionPoint = middle;
@@ -174,6 +178,8 @@ size_t LowerBound(const Container& aContainer, size_t aBegin, size_t aEnd,
     // Allow any intermediate type so long as it provides a suitable ordering
     // relation.
     const auto result = aCompare(aContainer[middle]);
+    static_assert(!std::is_unsigned_v<decltype(result)>,
+                  "comparator result must allow negative values");
 
     // The range returning from LowerBound does include elements
     // equivalent to the given value i.e. aCompare(element) == 0
@@ -200,6 +206,8 @@ size_t UpperBound(const Container& aContainer, size_t aBegin, size_t aEnd,
     // Allow any intermediate type so long as it provides a suitable ordering
     // relation.
     const auto result = aCompare(aContainer[middle]);
+    static_assert(!std::is_unsigned_v<decltype(result)>,
+                  "comparator result must allow negative values");
 
     // The range returning from UpperBound does NOT include elements
     // equivalent to the given value i.e. aCompare(element) == 0
@@ -226,6 +234,8 @@ std::pair<size_t, size_t> EqualRange(const Container& aContainer, size_t aBegin,
     // Allow any intermediate type so long as it provides a suitable ordering
     // relation.
     const auto result = aCompare(aContainer[middle]);
+    static_assert(!std::is_unsigned_v<decltype(result)>,
+                  "comparator result must allow negative values");
 
     if (result < 0) {
       high = middle;