diff --git a/src/core/channel/call_op_string.c b/src/core/channel/call_op_string.c
index 40d53693c22..127ea707bf5 100644
--- a/src/core/channel/call_op_string.c
+++ b/src/core/channel/call_op_string.c
@@ -84,7 +84,7 @@ char *grpc_call_op_string(grpc_call_op *op) {
       gpr_strvec_add(&b, gpr_strdup("SEND_MESSAGE"));
       break;
     case GRPC_SEND_PREFORMATTED_MESSAGE:
-      bprintf(&b, "SEND_PREFORMATTED_MESSAGE");
+      gpr_strvec_add(&b, gpr_strdup("SEND_PREFORMATTED_MESSAGE"));
       break;
     case GRPC_SEND_FINISH:
       gpr_strvec_add(&b, gpr_strdup("SEND_FINISH"));