diff --git a/src/pkg/runtime/string.go b/src/pkg/runtime/string.go
index 72b732f844..e9ea926dff 100644
--- a/src/pkg/runtime/string.go
+++ b/src/pkg/runtime/string.go
@@ -18,7 +18,7 @@ func concatstrings(a []string) string {
 			continue
 		}
 		if l+n < l {
-			panic("string concatenation too long")
+			gothrow("string concatenation too long")
 		}
 		l += n
 		count++