diff --git a/src/cfengine_cli/lint.py b/src/cfengine_cli/lint.py
index 47691d7..a23bc0f 100644
--- a/src/cfengine_cli/lint.py
+++ b/src/cfengine_cli/lint.py
@@ -492,6 +492,15 @@ def _lint_node(node: Node, policy_file: PolicyFile, state: State) -> int:
             f"Error: Bundle type must be one of ({', '.join(ALLOWED_BUNDLE_TYPES)}), not '{_text(node)}' {location}"
         )
         return 1
+    if state.strict and (
+        node.type in ("bundle_block_name", "body_block_name")
+        and _text(node) in BUILTIN_FUNCTIONS
+    ):
+        _highlight_range(node, lines)
+        print(
+            f"Error: {"Bundle" if "bundle" in node.type else "Body"} '{_text(node)}' conflicts with built-in function with the same name {location}"
+        )
+        return 1
     if node.type == "calling_identifier":
         name = _text(node)
         qualified_name = _qualify(name, state.namespace)
@@ -515,6 +524,26 @@ def _lint_node(node: Node, policy_file: PolicyFile, state: State) -> int:
                 f"Error: Call to unknown function / bundle / body '{name}' {location}"
             )
             return 1
+        if (
+            name not in BUILTIN_FUNCTIONS
+            and state.promise_type == "vars"
+            and state.attribute_name not in ("action", "classes")
+        ):
+            _highlight_range(node, lines)
+            print(
+                f"Error: Call to unknown function '{name}' inside 'vars'-promise {location}"
+            )
+            return 1
+        if (
+            state.promise_type == "vars"
+            and state.attribute_name in ("action", "classes")
+            and qualified_name not in state.bodies
+        ):
+            _highlight_range(node, lines)
+            print(
+                f"Error: '{name}' is not a defined body. Only bodies may be called with '{state.attribute_name}' {location}"
+            )
+            return 1
     return 0
 
 
diff --git a/tests/lint/009_bundle_shadows_function.cf b/tests/lint/009_bundle_shadows_function.cf
new file mode 100644
index 0000000..f06235a
--- /dev/null
+++ b/tests/lint/009_bundle_shadows_function.cf
@@ -0,0 +1,5 @@
+bundle agent isvariab
+{
+  reports:
+    "isvariab is not a built-in function";
+}
diff --git a/tests/lint/009_bundle_shadows_function.expected.txt b/tests/lint/009_bundle_shadows_function.expected.txt
new file mode 100644
index 0000000..4ad2733
--- /dev/null
+++ b/tests/lint/009_bundle_shadows_function.expected.txt
@@ -0,0 +1,6 @@
+
+bundle agent isvariable
+             ^--------^
+Error: Bundle 'isvariable' conflicts with built-in function with the same name at tests/lint/009_bundle_shadows_function.x.cf:1:14
+FAIL: tests/lint/009_bundle_shadows_function.x.cf (1 error)
+Failure, 1 error in total.
diff --git a/tests/lint/009_bundle_shadows_function.x.cf b/tests/lint/009_bundle_shadows_function.x.cf
new file mode 100644
index 0000000..81d46a3
--- /dev/null
+++ b/tests/lint/009_bundle_shadows_function.x.cf
@@ -0,0 +1,5 @@
+bundle agent isvariable
+{
+  reports:
+    "isvariable is a built-in function";
+}
diff --git a/tests/lint/010_unknown_function_inside_vars.cf b/tests/lint/010_unknown_function_inside_vars.cf
new file mode 100644
index 0000000..afd0bcd
--- /dev/null
+++ b/tests/lint/010_unknown_function_inside_vars.cf
@@ -0,0 +1,22 @@
+body classes bundle_class(a)
+{
+  scope => "bundle";
+  promise_kept => { "$(a):bundle_context" };
+}
+
+bundle agent target(a)
+{
+  reports:
+    "Hello, $(a)";
+}
+
+bundle agent helper
+{
+  vars:
+    "x" string => isvariable("arg");
+    "y"
+      classes => bundle_class("arg"),
+      string => "";
+  methods:
+    "x" usebundle => target("arg");
+}
diff --git a/tests/lint/010_unknown_function_inside_vars.expected.txt b/tests/lint/010_unknown_function_inside_vars.expected.txt
new file mode 100644
index 0000000..ce55a44
--- /dev/null
+++ b/tests/lint/010_unknown_function_inside_vars.expected.txt
@@ -0,0 +1,12 @@
+
+  vars:
+    "x" string => target("arg");
+                  ^----^
+Error: Call to unknown function 'target' inside 'vars'-promise at tests/lint/010_unknown_function_inside_vars.x.cf:16:19
+
+    "y"
+      classes => isvariable("arg"),
+                 ^--------^
+Error: 'isvariable' is not a defined body. Only bodies may be called with 'classes' at tests/lint/010_unknown_function_inside_vars.x.cf:18:18
+FAIL: tests/lint/010_unknown_function_inside_vars.x.cf (2 errors)
+Failure, 2 errors in total.
diff --git a/tests/lint/010_unknown_function_inside_vars.x.cf b/tests/lint/010_unknown_function_inside_vars.x.cf
new file mode 100644
index 0000000..cfeadca
--- /dev/null
+++ b/tests/lint/010_unknown_function_inside_vars.x.cf
@@ -0,0 +1,22 @@
+body classes bundle_class(a)
+{
+  scope => "bundle";
+  promise_kept => { "$(a):bundle_context" };
+}
+
+bundle agent target(a)
+{
+  reports:
+    "Hello, $(a)";
+}
+
+bundle agent helper
+{
+  vars:
+    "x" string => target("arg");
+    "y"
+      classes => isvariable("arg"),
+      string => "";
+  methods:
+    "x" usebundle => target("arg");
+}