diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index e4937735b..876ba32fa 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1730,7 +1730,7 @@ void EvalState::incrFunctionCall(ExprLambda * fun)
 }
 
 
-void EvalState::autoCallFunction(Bindings & args, Value & fun, Value & res)
+void EvalState::autoCallFunction(const Bindings & args, Value & fun, Value & res)
 {
     auto pos = fun.determinePos(noPos);
 
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index f7ed6be83..3171bf7fa 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -693,7 +693,7 @@ public:
      * Automatically call a function for which each argument has a
      * default value or has a binding in the `args` map.
      */
-    void autoCallFunction(Bindings & args, Value & fun, Value & res);
+    void autoCallFunction(const Bindings & args, Value & fun, Value & res);
 
     /**
      * Allocation primitives.