diff --git a/crates/emmylua_code_analysis/src/check/test/mod.rs b/crates/emmylua_code_analysis/src/check/test/mod.rs
index 3020a1014..0fb638cb0 100644
--- a/crates/emmylua_code_analysis/src/check/test/mod.rs
+++ b/crates/emmylua_code_analysis/src/check/test/mod.rs
@@ -44,6 +44,7 @@ mod unbalanced_assignments_test;
 mod undefined_doc_param_test;
 mod undefined_field_test;
 mod undefined_global_test;
+mod union_member_argument_test;
 mod unknown_doc_tag_test;
 mod unnecessary_assert_test;
 mod unnecessary_if_test;
diff --git a/crates/emmylua_code_analysis/src/check/test/union_member_argument_test.rs b/crates/emmylua_code_analysis/src/check/test/union_member_argument_test.rs
new file mode 100644
index 000000000..6f5247588
--- /dev/null
+++ b/crates/emmylua_code_analysis/src/check/test/union_member_argument_test.rs
@@ -0,0 +1,244 @@
+use crate::{DiagnosticCode, VirtualWorkspace};
+use googletest::prelude::*;
+use lsp_types::{Diagnostic, NumberOrString};
+use tokio_util::sync::CancellationToken;
+
+// Each test gets a fresh workspace; these shared classes are defined exactly once.
+const TYPES: &str = r#"
+---@alias uint32 integer
+---@class OneGoal
+---@field kind "one"
+---@field start_index? uint32
+---@class AllGoals
+---@field kind "all"
+---@field goals integer[]
+---@return OneGoal|AllGoals
+local function request_path() end
+---@return OneGoal
+local function request_one() end
+---@param index integer
+local function accept_index(index) end
+"#;
+
+// Negative cases inspect diagnostic counts and messages, not just their presence.
+fn mismatches(source: &str) -> Vec<Diagnostic> {
+    let mut ws = VirtualWorkspace::new_with_init_std_lib();
+    ws.analysis
+        .diagnostic
+        .enable_only(DiagnosticCode::ParamTypeMismatch);
+    let file = ws.def(&format!("{TYPES}\n{source}"));
+    let diagnostics = ws
+        .analysis
+        .diagnose_file(file, CancellationToken::new())
+        .expect("parameter diagnostics");
+    let code = Some(NumberOrString::String(
+        DiagnosticCode::ParamTypeMismatch.get_name().to_owned(),
+    ));
+    assert!(diagnostics.iter().all(|diagnostic| diagnostic.code == code));
+    diagnostics
+}
+
+#[gtest]
+fn cast_owner_before_passing_optional_member() {
+    let source = r#"
+        ---@param values string[]
+        local function remove_value(values)
+            local result = request_path()
+            ---@cast result OneGoal
+            ---@cast result.start_index uint32
+            table.remove(values, result.start_index)
+            if values[1] then values[2] = "updated" end
+            table.remove(values, result.start_index)
+        end
+        "#;
+    let mut ws = VirtualWorkspace::new_with_init_std_lib();
+    assert!(ws.has_no_diagnostic(
+        DiagnosticCode::ParamTypeMismatch,
+        &format!("{TYPES}\n{source}"),
+    ));
+}
+
+#[gtest]
+fn guard_and_assignment_narrow_member_owner() {
+    let source = r#"
+        local result = request_path()
+        if result.kind == "one" then
+            ---@cast result.start_index uint32
+            accept_index(result.start_index)
+        end
+        result = request_one()
+        ---@cast result.start_index uint32
+        accept_index(result.start_index)
+        "#;
+    let mut ws = VirtualWorkspace::new_with_init_std_lib();
+    assert!(ws.has_no_diagnostic(
+        DiagnosticCode::ParamTypeMismatch,
+        &format!("{TYPES}\n{source}"),
+    ));
+}
+
+#[gtest]
+fn narrowed_owner_union_checks_only_remaining_components() {
+    let source = r#"
+        ---@class AnotherGoal
+        ---@field start_index uint32
+        ---@param result OneGoal|AllGoals|AnotherGoal
+        ---@param values string[]
+        local function narrowed_union(result, values)
+            ---@cast result OneGoal|AnotherGoal
+            table.remove(values, result.start_index)
+        end
+        "#;
+    let mut ws = VirtualWorkspace::new_with_init_std_lib();
+    assert!(ws.has_no_diagnostic(
+        DiagnosticCode::ParamTypeMismatch,
+        &format!("{TYPES}\n{source}"),
+    ));
+}
+
+#[gtest]
+fn nested_owner_cast_is_respected() {
+    let source = r#"
+        ---@class Wrapper
+        ---@field result OneGoal|AllGoals
+        ---@param wrapper Wrapper
+        ---@param values string[]
+        local function remove_value(wrapper, values)
+            ---@cast wrapper.result OneGoal
+            table.remove(values, wrapper.result.start_index)
+        end
+        "#;
+    let mut ws = VirtualWorkspace::new_with_init_std_lib();
+    assert!(ws.has_no_diagnostic(
+        DiagnosticCode::ParamTypeMismatch,
+        &format!("{TYPES}\n{source}"),
+    ));
+}
+
+#[gtest]
+fn unnarrowed_union_still_reports_missing_member() {
+    let diagnostics = mismatches(
+        r#"
+        local result = request_path()
+        accept_index(result.start_index)
+        "#,
+    );
+    assert_that!(diagnostics.len(), eq(1));
+    assert_that!(diagnostics[0].message, contains_substring("nil"));
+}
+
+#[gtest]
+fn branch_guard_does_not_hide_missing_members_on_other_paths() {
+    let diagnostics = mismatches(
+        r#"
+        local result = request_path()
+        if result.kind == "one" then
+            ---@cast result.start_index uint32
+            accept_index(result.start_index)
+        else
+            accept_index(result.start_index)
+        end
+        accept_index(result.start_index)
+        "#,
+    );
+    assert_that!(diagnostics.len(), eq(2));
+}
+
+#[gtest]
+fn narrowed_owner_still_checks_the_actual_field_type() {
+    let diagnostics = mismatches(
+        r#"
+        ---@param text string
+        local function accept_text(text) end
+        local result = request_path()
+        ---@cast result OneGoal
+        ---@cast result.start_index uint32
+        accept_text(result.start_index)
+        "#,
+    );
+    assert_that!(diagnostics.len(), eq(1));
+    assert_that!(diagnostics[0].message, contains_substring("string"));
+}
+
+#[gtest]
+fn cast_can_widen_non_union_owner_to_union_with_missing_member() {
+    let diagnostics = mismatches(
+        r#"
+        ---@param result OneGoal
+        local function widened_owner(result)
+            ---@cast result OneGoal|AllGoals
+            accept_index(result.start_index)
+        end
+        "#,
+    );
+    assert_that!(diagnostics.len(), eq(1));
+    assert_that!(diagnostics[0].message, contains_substring("nil"));
+}
+
+#[gtest]
+fn cast_to_unrelated_owner_with_incompatible_field_is_rejected() {
+    let diagnostics = mismatches(
+        r#"
+        ---@class TextGoal
+        ---@field start_index string
+        local result = request_path()
+        ---@cast result TextGoal
+        accept_index(result.start_index)
+        "#,
+    );
+    assert_that!(diagnostics.len(), eq(1));
+}
+
+#[gtest]
+fn wide_union_cast_keeps_only_selected_owner() {
+    let mut source = String::new();
+    let mut types = vec!["OneGoal".to_owned()];
+    for index in 0..16 {
+        source.push_str(&format!(
+            "---@class OtherGoal{index}\n---@field other{index} boolean\n"
+        ));
+        types.push(format!("OtherGoal{index}"));
+    }
+    source.push_str(&format!(
+        r#"
+        ---@param result {}
+        ---@param values string[]
+        local function wide_union(result, values)
+            ---@cast result OneGoal
+            table.remove(values, result.start_index)
+        end
+        "#,
+        types.join("|")
+    ));
+    let mut ws = VirtualWorkspace::new_with_init_std_lib();
+    assert!(ws.has_no_diagnostic(
+        DiagnosticCode::ParamTypeMismatch,
+        &format!("{TYPES}\n{source}"),
+    ));
+}
+
+#[test]
+#[ntest::timeout(5000)]
+fn recursive_class_union_member_queries_terminate() {
+    let prefix = format!("node{}", ".next".repeat(16));
+    let source = format!(
+        r#"
+        ---@class WithIndex
+        ---@field next WithIndex|WithoutIndex
+        ---@field start_index integer
+        ---@class WithoutIndex
+        ---@field next WithIndex|WithoutIndex
+        ---@field other boolean
+        ---@param node WithIndex|WithoutIndex
+        local function recursive_owner(node)
+            ---@cast {prefix} WithIndex
+            accept_index({prefix}.start_index)
+        end
+        "#
+    );
+    let mut ws = VirtualWorkspace::new_with_init_std_lib();
+    assert!(ws.has_no_diagnostic(
+        DiagnosticCode::ParamTypeMismatch,
+        &format!("{TYPES}\n{source}"),
+    ));
+}
diff --git a/crates/emmylua_code_analysis/src/semantic_model/access.rs b/crates/emmylua_code_analysis/src/semantic_model/access.rs
index 3c4683eb4..1d03df6c4 100644
--- a/crates/emmylua_code_analysis/src/semantic_model/access.rs
+++ b/crates/emmylua_code_analysis/src/semantic_model/access.rs
@@ -519,7 +519,10 @@ impl<'db> SemanticModel<'db> {
         let Some(prefix) = index_expr.get_prefix_expr() else {
             return false;
         };
-        let prefix_ty = self.prefix_type_for_member_resolution(&prefix);
+        // This argument-only check needs the owner after casts, guards and assignments.
+        // type_of_expr_at caches by syntax/offset. Do not skip declared non-unions:
+        // casts and assignments can widen them into unions with missing members.
+        let prefix_ty = self.type_of_expr_at(prefix.get_syntax_id(), index_expr.get_position());
         let LuaType::Union(union) = &prefix_ty else {
             return false;
         };