diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 18b7ef701..6dd3803d8 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -480,9 +480,8 @@ void EvalState::eval(Expr * e, Value & v)
 }
 
 
-inline bool EvalState::evalBool(Env & env, Expr * e)
+inline bool EvalState::evalBool(Env & env, Expr * e, Value & v)
 {
-    Value v;
     e->eval(*this, env, v);
     if (v.type != tBool)
         throwTypeError("value is %1% while a Boolean was expected", v);
@@ -490,6 +489,13 @@ inline bool EvalState::evalBool(Env & env, Expr * e)
 }
 
 
+inline bool EvalState::evalBool(Env & env, Expr * e)
+{
+    Value v;
+    return evalBool(env, e, v);
+}
+
+
 inline void EvalState::evalAttrs(Env & env, Expr * e, Value & v)
 {
     e->eval(*this, env, v);
@@ -838,13 +844,13 @@ void ExprWith::eval(EvalState & state, Env & env, Value & v)
 
 void ExprIf::eval(EvalState & state, Env & env, Value & v)
 {
-    (state.evalBool(env, cond) ? then : else_)->eval(state, env, v);
+    (state.evalBool(env, cond, v) ? then : else_)->eval(state, env, v);
 }
 
 
 void ExprAssert::eval(EvalState & state, Env & env, Value & v)
 {
-    if (!state.evalBool(env, cond))
+    if (!state.evalBool(env, cond, v))
         throwAssertionError("assertion failed at %1%", pos);
     body->eval(state, env, v);
 }
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index cac8af877..4e8e7e5f9 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -145,6 +145,7 @@ public:
 
     /* Evaluation the expression, then verify that it has the expected
        type. */
+    inline bool evalBool(Env & env, Expr * e, Value & v);
     inline bool evalBool(Env & env, Expr * e);
     inline void evalAttrs(Env & env, Expr * e, Value & v);