diff --git a/include/nextpow2.h b/include/nextpow2.h
index 075c5a88..e1e6ac18 100644
--- a/include/nextpow2.h
+++ b/include/nextpow2.h
@@ -12,6 +12,12 @@
 
 size_t nextpow2(size_t x) {
     assert(x != 0);
+#if defined(UINT64_MAX) && UINTPTR_MAX == UINT32_MAX
+    assert(x <= 1073741824);
+#endif
+#if defined(UINT64_MAX) && UINTPTR_MAX == UINT64_MAX
+    assert(x <= 4611686018427387904);
+#endif
 
 #ifdef __GNUC__
 #if __GNUC__ > 3 || (__GNUC__ == 3 && (__GNUC_MINOR__ > 4 || __GNUC_MINOR__ == 4)) // Test for GCC >= 3.4.0
@@ -23,7 +29,7 @@ size_t nextpow2(size_t x) {
 #endif
 #endif
 
-#if UINTPTR_MAX == UINT32_MAX || UINTPTR_MAX == UINT64_MAX
+#if defined(UINT32_MAX) && (UINTPTR_MAX == UINT32_MAX || UINTPTR_MAX == UINT64_MAX)
     x--;
     x |= x >> 1;
     x |= x >> 2;