diff --git a/crates/js/src/builtins_tests.rs b/crates/js/src/builtins_tests.rs
index 9d917ae32..0a70e8043 100644
--- a/crates/js/src/builtins_tests.rs
+++ b/crates/js/src/builtins_tests.rs
@@ -613,7 +613,11 @@ fn math_random_is_seeded_from_the_os_random_source() {
     let Some(start) = src.find("fn random_seed()") else { panic!("interp.rs has no random_seed") };
     let body = &src[start..];
     let body = &body[..body.find("\n}").unwrap_or(body.len())];
-    assert!(body.contains("getrandom::u64()"), "Math.random isn't seeded from the OS random source");
+    assert!(body.contains("random_seed_from(getrandom::u64())"), "Math.random isn't seeded from the OS random source");
+    // The OS value is the seed, made odd so it's never zero; a failed read still gives a seed.
+    assert_eq!(crate::interp::random_seed_from(Ok(0x40)), 0x41);
+    assert_eq!(crate::interp::random_seed_from(Ok(0)), 1);
+    assert_eq!(crate::interp::random_seed_from(Err(getrandom::Error::UNSUPPORTED)) & 1, 1);
 }
 
 // ---- JSON ----
diff --git a/crates/js/src/interp.rs b/crates/js/src/interp.rs
index 5d59d92d2..4f6a78a39 100644
--- a/crates/js/src/interp.rs
+++ b/crates/js/src/interp.rs
@@ -311,8 +311,13 @@ fn index_keys_bytes(len: usize) -> usize {
 /// random source (the browser's crypto on the web), or the standard library's randomly keyed
 /// hasher when that fails; never zero.
 fn random_seed() -> u64 {
+    random_seed_from(getrandom::u64())
+}
+
+/// [`random_seed`] from what the OS random source returned.
+pub(crate) fn random_seed_from(os: Result<u64, getrandom::Error>) -> u64 {
     use std::hash::{BuildHasher, Hasher};
-    getrandom::u64().unwrap_or_else(|_| {
+    os.unwrap_or_else(|_| {
         let mut h = std::collections::hash_map::RandomState::new().build_hasher();
         h.write_u64(0x9e37_79b9_7f4a_7c15);
         h.finish()