diff --git a/cutils.h b/cutils.h
index ff2d3fb62..11246e3ce 100644
--- a/cutils.h
+++ b/cutils.h
@@ -26,6 +26,7 @@
 #define CUTILS_H
 
 #include <stdlib.h>
+#include <string.h>
 #include <inttypes.h>
 
 #define likely(x)       __builtin_expect(!!(x), 1)
@@ -64,6 +65,12 @@ char *pstrcat(char *buf, int buf_size, const char *s);
 int strstart(const char *str, const char *val, const char **ptr);
 int has_suffix(const char *str, const char *suffix);
 
+/* Prevent UB when n == 0 and (src == NULL or dest == NULL) */
+static inline void memcpy_no_ub(void *dest, const void *src, size_t n) {
+    if (n)
+        memcpy(dest, src, n);
+}
+
 static inline int max_int(int a, int b)
 {
     if (a > b)
diff --git a/quickjs.c b/quickjs.c
index b5634fb42..ebf45a988 100644
--- a/quickjs.c
+++ b/quickjs.c
@@ -33390,8 +33390,8 @@ static JSValue js_create_function(JSContext *ctx, JSFunctionDef *fd)
             }
         } else {
             b->vardefs = (void *)((uint8_t*)b + vardefs_offset);
-            memcpy(b->vardefs, fd->args, fd->arg_count * sizeof(fd->args[0]));
-            memcpy(b->vardefs + fd->arg_count, fd->vars, fd->var_count * sizeof(fd->vars[0]));
+            memcpy_no_ub(b->vardefs, fd->args, fd->arg_count * sizeof(fd->args[0]));
+            memcpy_no_ub(b->vardefs + fd->arg_count, fd->vars, fd->var_count * sizeof(fd->vars[0]));
         }
         b->var_count = fd->var_count;
         b->arg_count = fd->arg_count;
@@ -53999,9 +53999,10 @@ static JSValue js_typed_array_indexOf(JSContext *ctx, JSValueConst this_val,
     } else
     if (tag == JS_TAG_FLOAT64) {
         d = JS_VALUE_GET_FLOAT64(argv[0]);
-        // XXX: should fix UB
-        v64 = d;
-        is_int = (v64 == d);
+        if (d >= INT64_MIN && d < 0x1p63) {
+            v64 = d;
+            is_int = (v64 == d);
+        }
     } else if (tag == JS_TAG_BIG_INT) {
         JSBigFloat *p1 = JS_VALUE_GET_PTR(argv[0]);