diff --git a/lix/libexpr/eval-inline.hh b/lix/libexpr/eval-inline.hh
index 3d414928d..a11df926e 100644
--- a/lix/libexpr/eval-inline.hh
+++ b/lix/libexpr/eval-inline.hh
@@ -20,11 +20,21 @@ inline Value::Value(app_t, EvalMemory & mem, Value & lhs, Value & rhs)
 }
 
 inline Value::Value(app_t, EvalMemory & mem, Value & lhs, std::span<Value> args)
+    : Value(app_t{}, mem, lhs, args, {})
 {
-    auto app = static_cast<Value::App *>(mem.allocBytes(sizeof(Value::App) + args.size_bytes()));
+}
+
+inline Value::Value(
+    app_t, EvalMemory & mem, const Value & lhs, std::span<Value> baseArgs, std::span<Value> moreArgs
+)
+{
+    auto app = static_cast<Value::App *>(
+        mem.allocBytes(sizeof(Value::App) + baseArgs.size_bytes() + moreArgs.size_bytes())
+    );
     app->_left = lhs;
-    app->_n = args.size();
-    std::copy(args.begin(), args.end(), app->_args);
+    app->_n = baseArgs.size() + moreArgs.size();
+    std::copy(baseArgs.begin(), baseArgs.end(), app->_args);
+    std::copy(moreArgs.begin(), moreArgs.end(), app->_args + baseArgs.size());
     raw = tag(tApp, app);
 }
 
diff --git a/lix/libexpr/eval.cc b/lix/libexpr/eval.cc
index d532d5da9..4e12801c3 100644
--- a/lix/libexpr/eval.cc
+++ b/lix/libexpr/eval.cc
@@ -1639,7 +1639,14 @@ void EvalState::callFunction(Value & fun, std::span<Value> args, Value & vRes, c
 
     Value vCur(fun);
 
-    auto makeAppChain = [&]() { vRes = {NewValueAs::app, ctx.mem, vCur, args}; };
+    auto makeAppChain = [&]() {
+        if (vCur.isApp()) {
+            auto & app = vCur.app();
+            vRes = {NewValueAs::app, ctx.mem, app.left(), app.args(), args};
+        } else {
+            vRes = {NewValueAs::app, ctx.mem, vCur, args};
+        }
+    };
 
     const Attr * functor;
 
diff --git a/lix/libexpr/value.hh b/lix/libexpr/value.hh
index 1f3d80f8f..75a16fae6 100644
--- a/lix/libexpr/value.hh
+++ b/lix/libexpr/value.hh
@@ -544,6 +544,10 @@ public:
     /// lazy and/or partial application of a function.
     Value(app_t, EvalMemory & mem, Value & lhs, std::span<Value> args);
 
+    /// Constructs a nix language value of type "lambda", which represents a
+    /// lazy and/or partial application of a function.
+    Value(app_t, EvalMemory & mem, const Value & lhs, std::span<Value> baseArgs, std::span<Value> moreArgs);
+
     /// Constructs a nix language value of type "external", which is only used
     /// by plugins. Do any existing plugins even use this mechanism?
     Value(external_t, ExternalValueBase & external)
diff --git a/tests/functional2/lang/call-primop/eval-fail.err.exp b/tests/functional2/lang/call-primop/eval-fail-incomplete.err.exp
similarity index 100%
rename from tests/functional2/lang/call-primop/eval-fail.err.exp
rename to tests/functional2/lang/call-primop/eval-fail-incomplete.err.exp
diff --git a/tests/functional2/lang/call-primop/eval-okay-app-extension.out.exp b/tests/functional2/lang/call-primop/eval-okay-app-extension.out.exp
new file mode 100644
index 000000000..ce291fa51
--- /dev/null
+++ b/tests/functional2/lang/call-primop/eval-okay-app-extension.out.exp
@@ -0,0 +1 @@
+"234"
diff --git a/tests/functional2/lang/call-primop/in-app-extension.nix b/tests/functional2/lang/call-primop/in-app-extension.nix
new file mode 100644
index 000000000..bc627c0f2
--- /dev/null
+++ b/tests/functional2/lang/call-primop/in-app-extension.nix
@@ -0,0 +1,5 @@
+let
+  a = builtins.substring 1;
+  b = a 3;
+in
+b "1234567890"
diff --git a/tests/functional2/lang/call-primop/in.nix b/tests/functional2/lang/call-primop/in-incomplete.nix
similarity index 100%
rename from tests/functional2/lang/call-primop/in.nix
rename to tests/functional2/lang/call-primop/in-incomplete.nix