diff --git a/internal/wwfixture/run.ww b/internal/wwfixture/run.ww
index 963960b4..86fdceea 100644
--- a/internal/wwfixture/run.ww
+++ b/internal/wwfixture/run.ww
@@ -423,6 +423,10 @@ fn cancelall(handles: []exec.process) bool = {
 };
 
 fn runfixtures(opts: *options, c: *corpus, ids: []identity) int = {
+	if (ids.len > 9999) {
+		fmt.errorln("wwfixture: selected matrix exceeds four-digit capture namespace");
+		return 1;
+	};
 	let runroot: str = strings.dup(temp.dir());
 	let rid: str = runid(runroot);
 	let cells: []fixturecell;