diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index ce87eaed2f..035151bb26 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -3141,6 +3141,8 @@ void Worker::waitForInput()
         }
     }
 
+    assert(fdMax != 0);
+
     if (select(fdMax, &fds, 0, 0, useTimeout ? &timeout : 0) == -1) {
         if (errno == EINTR) return;
         throw SysError("waiting for input");