diff --git a/nix-repl.cc b/nix-repl.cc
index 474ad4fc9..1077f5d8f 100644
--- a/nix-repl.cc
+++ b/nix-repl.cc
@@ -250,12 +250,14 @@ static int runProgram(const string & program, const Strings & args)
 
 bool isVarName(const string & s)
 {
-    // FIXME: not quite correct.
+    if (s.size() == 0) return false;
+    char c = s[0];
+    if ((c >= '0' && c <= '9') || c == '-' || c == '\'') return false;
     for (auto & i : s)
         if (!((i >= 'a' && i <= 'z') ||
               (i >= 'A' && i <= 'Z') ||
               (i >= '0' && i <= '9') ||
-              i == '_' || i == '\''))
+              i == '_' || i == '-' || i == '\''))
             return false;
     return true;
 }