diff --git a/src/wutil.cpp b/src/wutil.cpp
index bbbbe497b..9aa76102b 100644
--- a/src/wutil.cpp
+++ b/src/wutil.cpp
@@ -625,6 +625,37 @@ long long fish_wcstoll(const wchar_t *str, const wchar_t **endptr, int base) {
     return result;
 }
 
+/// An enhanced version of wcstoull().
+///
+/// This is needed because BSD and GNU implementations differ in several ways that make it really
+/// annoying to use them in a portable fashion.
+///
+/// The caller doesn't have to zero errno. Sets errno to -1 if the int ends with something other
+/// than a digit. Leading whitespace is ignored (per the base wcstoll implementation). Trailing
+/// whitespace is also ignored.
+unsigned long long fish_wcstoull(const wchar_t *str, const wchar_t **endptr, int base) {
+    while (iswspace(*str)) ++str;  // skip leading whitespace
+    if (!*str) {  // this is because some implementations don't handle this sensibly
+        errno = EINVAL;
+        if (endptr) *endptr = str;
+        return 0;
+    }
+
+    errno = 0;
+    wchar_t *_endptr;
+    unsigned long long result = wcstoull(str, &_endptr, base);
+    while (iswspace(*_endptr)) ++_endptr;  // skip trailing whitespace
+    if (!errno && *_endptr) {
+        if (_endptr == str) {
+            errno = EINVAL;
+        } else {
+            errno = -1;
+        }
+    }
+    if (endptr) *endptr = _endptr;
+    return result;
+}
+
 file_id_t file_id_t::file_id_from_stat(const struct stat *buf) {
     assert(buf != NULL);
 
diff --git a/src/wutil.h b/src/wutil.h
index a529de9ce..2bd8ed80a 100644
--- a/src/wutil.h
+++ b/src/wutil.h
@@ -120,6 +120,7 @@ int fish_wcswidth(const wcstring &str);
 int fish_wcstoi(const wchar_t *str, const wchar_t **endptr = NULL, int base = 10);
 long fish_wcstol(const wchar_t *str, const wchar_t **endptr = NULL, int base = 10);
 long long fish_wcstoll(const wchar_t *str, const wchar_t **endptr = NULL, int base = 10);
+unsigned long long fish_wcstoull(const wchar_t *str, const wchar_t **endptr = NULL, int base = 10);
 
 /// Class for representing a file's inode. We use this to detect and avoid symlink loops, among
 /// other things. While an inode / dev pair is sufficient to distinguish co-existing files, Linux