diff --git a/src/libexpr/common-opts.cc b/src/libexpr/common-opts.cc
index 5a4856568..9131951e3 100644
--- a/src/libexpr/common-opts.cc
+++ b/src/libexpr/common-opts.cc
@@ -23,7 +23,7 @@ bool parseOptionArg(const string & arg, Strings::iterator & i,
     Value & v(autoArgs[state.symbols.create(name)].value);
 
     if (arg == "--arg")
-        state.mkThunk_( v, parseExprFromString(state, value, absPath(".")));
+        state.mkThunk_(v, parseExprFromString(state, value, absPath(".")));
     else
         mkString(v, value);
     
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 14e5cab7d..1ee6c5a7f 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -16,7 +16,7 @@
 namespace nix {
     
 
-std::ostream & operator << (std::ostream & str, Value & v)
+std::ostream & operator << (std::ostream & str, const Value & v)
 {
     switch (v.type) {
     case tInt:
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 0f2cb6c13..4e0af70ba 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -173,7 +173,7 @@ typedef std::map<Path, Path> SrcToStore;
 struct EvalState;
 
 
-std::ostream & operator << (std::ostream & str, Value & v);
+std::ostream & operator << (std::ostream & str, const Value & v);
 
 
 class EvalState