diff --git a/src/builtins/array-join.tq b/src/builtins/array-join.tq
index 84b967fa597..9d107f2b4e0 100644
--- a/src/builtins/array-join.tq
+++ b/src/builtins/array-join.tq
@@ -203,6 +203,11 @@ struct Buffer {
       implicit context: Context)(nofSeparators: intptr,
       separatorLength: intptr, write: bool): void {
     if (nofSeparators == 0 || separatorLength == 0) return;
+    // nofSeparators is the unguarded sum of up to kMaxArrayLength increments.
+    // On 32-bit targets intptr is int32 and the count can wrap negative; treat
+    // that as length overflow instead of letting the negative value reach the
+    // chunk-list (where WriteChunkListToFlat reads it as repeat_last).
+    if (nofSeparators < 0) ThrowInvalidStringLength(context);
 
     const nofSeparatorsInt: intptr = nofSeparators;
     const sepsLen: intptr = separatorLength * nofSeparatorsInt;