Use $Restrict

src/vmod_zipflow.c

@@ -123,10 +123,7 @@ vmod_set_level(VRT_CTX, VCL_INT level)
 		default_level = level;
 		return;
 	}
-	if (ctx->method & VCL_MET_TASK_H) {
-		VRT_fail(ctx, "set_level can not be called here");
-		return;
-	}
+	AZ(ctx->method & VCL_MET_TASK_H);
 
 	AN(ctx->method & (VCL_MET_TASK_C | VCL_MET_TASK_B));
 	zft = get_zipflow_task(ctx);

src/vmod_zipflow.vcc

@@ -39,6 +39,8 @@ Example
 
 $Function VOID set_level(INT level)
 
+$Restrict vcl_init client backend
+
 Set the (default) compression level for the zipflow filter.
 
 Valid values for `level` are -1 to select the default compression