diff --git a/test/test_iomux.c b/test/test_iomux.c
index ed4f10b..303a284 100644
--- a/test/test_iomux.c
+++ b/test/test_iomux.c
@@ -28,6 +28,7 @@ static void simple_rcb(struct IOMux *mux, struct IOMuxFD *fd, unsigned int event
 
     ck_assert_int_le(fd->cookie_int, 2);
     fd->cookie_int++;
+    fail_if(event != MUX_EV_READ);
 
     r = read(fd->fd, buf, sizeof buf);
     ck_assert(r >= 0);
@@ -53,6 +54,8 @@ static void simple_wcb(struct IOMux *mux, struct IOMuxFD *fd, unsigned int event
     ck_assert_int_eq(fd->cookie_int, 0);
     fd->cookie_int = 1;
 
+    fail_if(event != MUX_EV_WRITE);
+
     written = write(fd->fd, "Hello", 5);
     ck_assert_int_eq(written, 5);