diff --git a/js/src/jsapi.cpp b/js/src/jsapi.cpp
index 417f5fef5a41..d8ce842e7de9 100644
--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -124,6 +124,14 @@ using JS::SourceText;
 #  define JS_ADDRESSOF_VA_LIST(ap) (&(ap))
 #endif
 
+// See preprocessor definition of JS_BITS_PER_WORD in jstypes.h; make sure
+// JS_64BIT (used internally) agrees with it
+#ifdef JS_64BIT
+static_assert(JS_BITS_PER_WORD == 64, "values must be in sync");
+#else
+static_assert(JS_BITS_PER_WORD == 32, "values must be in sync");
+#endif
+
 JS_PUBLIC_API void JS::CallArgs::reportMoreArgsNeeded(JSContext* cx,
                                                       const char* fnname,
                                                       unsigned required,
diff --git a/js/src/jstypes.h b/js/src/jstypes.h
index 8d24a3aaf02c..f3881456301c 100644
--- a/js/src/jstypes.h
+++ b/js/src/jstypes.h
@@ -109,12 +109,17 @@ constexpr size_t RoundDown(size_t x, size_t y) { return (x / y) * y; }
 constexpr size_t Round(size_t x, size_t y) { return ((x + y / 2) / y) * y; }
 }  // namespace js
 
-#if defined(JS_64BIT)
+#if (defined(__SIZEOF_POINTER__) && __SIZEOF_POINTER__ == 8) || \
+    (defined(UINTPTR_MAX) && UINTPTR_MAX == 0xFFFFFFFFFFFFFFFFu)
 #  define JS_BITS_PER_WORD 64
 #else
 #  define JS_BITS_PER_WORD 32
 #endif
 
+static_assert(sizeof(void*) == 8 ? JS_BITS_PER_WORD == 64
+                                 : JS_BITS_PER_WORD == 32,
+              "preprocessor and compiler must agree");
+
 /***********************************************************************
 ** MACROS:      JS_FUNC_TO_DATA_PTR
 **              JS_DATA_TO_FUNC_PTR