diff --git a/src/sim/rng.js b/src/sim/rng.js
index eebeace..2b6d4a4 100644
--- a/src/sim/rng.js
+++ b/src/sim/rng.js
@@ -24,5 +24,7 @@ export function makeRng(seed) {
 // This is the whole hidden-variable mechanism: outcomes are fixed
 // the moment their label exists, revealed whenever first asked for.
 export function rollFor(seed, label) {
-  return makeRng(hash32(seed + ':' + label))();
+  // Length-prefix the seed so (seed, label) -> string is injective
+  // even though seeds and labels themselves contain ':'.
+  return makeRng(hash32(seed.length + ':' + seed + ':' + label))();
 }