diff --git a/src/libcmd/repl.cc b/src/libcmd/repl.cc
index df8932087..bb254ff8d 100644
--- a/src/libcmd/repl.cc
+++ b/src/libcmd/repl.cc
@@ -384,6 +384,10 @@ StringSet NixRepl::completePrefix(const std::string & prefix)
             i++;
         }
     } else {
+        /* Temporarily disable the debugger, to avoid re-entering readline. */
+        auto debug_repl = state->debugRepl;
+        state->debugRepl = nullptr;
+        Finally restoreDebug([&]() { state->debugRepl = debug_repl; });
         try {
             /* This is an expression that should evaluate to an
                attribute set.  Evaluate it to get the names of the