diff --git a/mfbt/TypeTraits.h b/mfbt/TypeTraits.h
index ab594fd4bb28..3184221ec598 100644
--- a/mfbt/TypeTraits.h
+++ b/mfbt/TypeTraits.h
@@ -417,26 +417,30 @@ namespace detail {
 
 template<typename T,
          bool = IsFloatingPoint<T>::value,
+         bool = IsIntegral<T>::value,
          typename NoCV = typename RemoveCV<T>::Type>
 struct IsSignedHelper;
 
+// Floating point is signed.
 template<typename T, typename NoCV>
-struct IsSignedHelper<T, true, NoCV> : TrueType {};
+struct IsSignedHelper<T, true, false, NoCV> : TrueType {};
 
+// Integral is conditionally signed.
 template<typename T, typename NoCV>
-struct IsSignedHelper<T, false, NoCV>
-  : IntegralConstant<bool, IsArithmetic<T>::value && NoCV(-1) < NoCV(1)>
+struct IsSignedHelper<T, false, true, NoCV>
+  : IntegralConstant<bool, bool(NoCV(-1) < NoCV(1))>
 {};
 
+// Non-floating point, non-integral is not signed.
+template<typename T, typename NoCV>
+struct IsSignedHelper<T, false, false, NoCV> : FalseType {};
+
 } // namespace detail
 
 /**
  * IsSigned determines whether a type is a signed arithmetic type.  |char| is
  * considered a signed type if it has the same representation as |signed char|.
  *
- * Don't use this if the type might be user-defined!  You might or might not get
- * a compile error, depending.
- *
  * mozilla::IsSigned<int>::value is true;
  * mozilla::IsSigned<const unsigned int>::value is false;
  * mozilla::IsSigned<unsigned char>::value is false;
@@ -449,27 +453,30 @@ namespace detail {
 
 template<typename T,
          bool = IsFloatingPoint<T>::value,
+         bool = IsIntegral<T>::value,
          typename NoCV = typename RemoveCV<T>::Type>
 struct IsUnsignedHelper;
 
+// Floating point is not unsigned.
 template<typename T, typename NoCV>
-struct IsUnsignedHelper<T, true, NoCV> : FalseType {};
+struct IsUnsignedHelper<T, true, false, NoCV> : FalseType {};
 
+// Integral is conditionally unsigned.
 template<typename T, typename NoCV>
-struct IsUnsignedHelper<T, false, NoCV>
+struct IsUnsignedHelper<T, false, true, NoCV>
   : IntegralConstant<bool,
-                     IsArithmetic<T>::value &&
-                     (IsSame<NoCV, bool>::value || NoCV(1) < NoCV(-1))>
+                     (IsSame<NoCV, bool>::value || bool(NoCV(1) < NoCV(-1)))>
 {};
 
+// Non-floating point, non-integral is not unsigned.
+template<typename T, typename NoCV>
+struct IsUnsignedHelper<T, false, false, NoCV> : FalseType {};
+
 } // namespace detail
 
 /**
  * IsUnsigned determines whether a type is an unsigned arithmetic type.
  *
- * Don't use this if the type might be user-defined!  You might or might not get
- * a compile error, depending.
- *
  * mozilla::IsUnsigned<int>::value is false;
  * mozilla::IsUnsigned<const unsigned int>::value is true;
  * mozilla::IsUnsigned<unsigned char>::value is true;
diff --git a/mfbt/tests/TestTypeTraits.cpp b/mfbt/tests/TestTypeTraits.cpp
index cc0057aed6e2..eef8a867f9fd 100644
--- a/mfbt/tests/TestTypeTraits.cpp
+++ b/mfbt/tests/TestTypeTraits.cpp
@@ -153,6 +153,16 @@ static_assert(IsSigned<const volatile long double>::value,
 static_assert(!IsUnsigned<const volatile long double>::value,
               "const volatile long double shouldn't be unsigned");
 
+class NotIntConstructible
+{
+  NotIntConstructible(int) MOZ_DELETE;
+};
+
+static_assert(!IsSigned<NotIntConstructible>::value,
+              "non-arithmetic types are not signed");
+static_assert(!IsUnsigned<NotIntConstructible>::value,
+              "non-arithmetic types are not unsigned");
+
 namespace CPlusPlus11IsBaseOf {
 
 // Adapted from C++11 § 20.9.6.