diff --git a/src/fix-ng/eval.cc b/src/fix-ng/eval.cc
index 1f131fb7b0..8e10caa843 100644
--- a/src/fix-ng/eval.cc
+++ b/src/fix-ng/eval.cc
@@ -137,12 +137,12 @@ Expr evalExpr2(EvalState & state, Expr e)
         ATmatch(e, "Bool(<term>)", &e1) ||
         ATmatch(e, "Function([<list>], <term>)", &e1, &e2) ||
         ATmatch(e, "Attrs([<list>])", &e1) ||
-        ATmatch(e, "List([<list>])", &e1) ||
-        ATmatch(e, "Null", &e1))
+        ATmatch(e, "List([<list>])", &e1))
         return e;
 
     /* Any encountered variables must be undeclared or primops. */
     if (ATmatch(e, "Var(<str>)", &s1)) {
+        if ((string) s1 == "null") return primNull(state);
         return e;
     }
 
@@ -159,7 +159,6 @@ Expr evalExpr2(EvalState & state, Expr e)
             if (primop == "derivation") return primDerivation(state, e2);
             if (primop == "toString") return primToString(state, e2);
             if (primop == "baseNameOf") return primBaseNameOf(state, e2);
-            if (primop == "null") return primNull(state, e2);
             if (primop == "isNull") return primIsNull(state, e2);
             else throw badTerm("undefined variable/primop", e1);
         }
diff --git a/src/fix-ng/primops.cc b/src/fix-ng/primops.cc
index f5a278f669..df5747edd4 100644
--- a/src/fix-ng/primops.cc
+++ b/src/fix-ng/primops.cc
@@ -227,7 +227,7 @@ Expr primToString(EvalState & state, Expr arg)
 }
 
 
-Expr primNull(EvalState & state, Expr arg)
+Expr primNull(EvalState & state)
 {
     return ATmake("Null");
 }
diff --git a/src/fix-ng/primops.hh b/src/fix-ng/primops.hh
index 775ec55682..76d587afdb 100644
--- a/src/fix-ng/primops.hh
+++ b/src/fix-ng/primops.hh
@@ -25,7 +25,7 @@ Expr primBaseNameOf(EvalState & state, Expr arg);
 Expr primToString(EvalState & state, Expr arg);
 
 /* Return the null value. */
-Expr primNull(EvalState & state, Expr arg);
+Expr primNull(EvalState & state);
 
 /* Determine whether the argument is the null value. */
 Expr primIsNull(EvalState & state, Expr arg);