diff --git a/src/iomux_impl.c b/src/iomux_impl.c
index f94e0c9..2f41aa6 100644
--- a/src/iomux_impl.c
+++ b/src/iomux_impl.c
@@ -116,6 +116,7 @@ int iomux_register_fd(struct IOMux *mux, struct IOMuxFD *fd)
     assert(mux != NULL);
     assert(fd != NULL);
     assert(fd->callback != NULL);
+    assert(fd->fd >= 0);
     return mux->register_fd_impl(mux, fd);
 }
 
@@ -123,6 +124,7 @@ int iomux_unregister_fd(struct IOMux *mux, struct IOMuxFD *fd)
 {
     assert(mux != NULL);
     assert(fd != NULL);
+    assert(fd->fd >= 0);
     return mux->unregister_fd_impl(mux, fd);
 }
 
@@ -131,6 +133,7 @@ int iomux_update_events(struct IOMux *mux, struct IOMuxFD *fd)
     assert(mux != NULL);
     assert(fd != NULL);
     assert(fd->callback != NULL);
+    assert(fd->fd >= 0);
     return mux->update_events_impl(mux, fd);
 }