diff --git a/src/wutil.cpp b/src/wutil.cpp
index 0fee1658e..b9601ba06 100644
--- a/src/wutil.cpp
+++ b/src/wutil.cpp
@@ -711,13 +711,14 @@ unsigned long long fish_wcstoull(const wchar_t *str, const wchar_t **endptr, int
 /// Like wcstod(), but wcstod() is enormously expensive on some platforms so this tries to have a
 /// fast path.
 double fish_wcstod(const wchar_t *str, wchar_t **endptr) {
-    // The "fast path." If we're all ASCII, use strtod().
+    // The "fast path." If we're all ASCII and we fit inline, use strtod().
     char narrow[128];
     size_t len = std::wcslen(str);
     size_t len_plus_0 = 1 + len;
-    auto is_ascii = [](wchar_t c) { return 0 <= c && c <= 127; };
-    if (len_plus_0 <= sizeof narrow && std::all_of(str, str + len, is_ascii)) {
+    auto is_digit = [](wchar_t c) { return '0' <= c && c <= '9'; };
+    if (len_plus_0 <= sizeof narrow && std::all_of(str, str + len, is_digit)) {
         // Fast path. Copy the string into a local buffer and run strtod() on it.
+        // We can ignore the locale-taking version because we are limited to ASCII digits.
         std::copy(str, str + len_plus_0, narrow);
         char *narrow_endptr = nullptr;
         double ret = strtod(narrow, endptr ? &narrow_endptr : nullptr);