diff --git a/src/pkg/runtime/heapdump.c b/src/pkg/runtime/heapdump.c
index db0178eb65..bc0fd49c0a 100644
--- a/src/pkg/runtime/heapdump.c
+++ b/src/pkg/runtime/heapdump.c
@@ -803,7 +803,7 @@ playgcprog(uintptr offset, uintptr *prog, void (*callback)(void*,uintptr,uintptr
 			prog += 4;
 			break;
 		default:
-			runtime·printf("%D\n", prog[0]);
+			runtime·printf("%D\n", (uint64)prog[0]);
 			runtime·throw("bad gc op");
 		}
 	}