diff --git a/crates/nu-cli/src/syntax_highlight.rs b/crates/nu-cli/src/syntax_highlight.rs
index d0d5d06431..a8878b23c0 100644
--- a/crates/nu-cli/src/syntax_highlight.rs
+++ b/crates/nu-cli/src/syntax_highlight.rs
@@ -84,6 +84,9 @@ impl Highlighter for NuHighlighter {
                     Style::new().fg(nu_ansi_term::Color::Yellow).bold(),
                     next_token,
                 )),
+                FlatShape::Flag => {
+                    output.push((Style::new().fg(nu_ansi_term::Color::Yellow), next_token))
+                }
                 FlatShape::Filepath => output.push((
                     Style::new().fg(nu_ansi_term::Color::Yellow).bold(),
                     next_token,
diff --git a/crates/nu-engine/src/eval.rs b/crates/nu-engine/src/eval.rs
index 6a642f7d50..4aa8465492 100644
--- a/crates/nu-engine/src/eval.rs
+++ b/crates/nu-engine/src/eval.rs
@@ -1,6 +1,6 @@
 use nu_protocol::ast::{Block, Call, Expr, Expression, Operator, Statement};
 use nu_protocol::engine::EvaluationContext;
-use nu_protocol::{Range, ShellError, Span, Type, Unit, Value};
+use nu_protocol::{Range, ShellError, Span, Spanned, Type, Unit, Value};
 
 pub fn eval_operator(op: &Expression) -> Result<Operator, ShellError> {
     match op {
@@ -60,6 +60,29 @@ fn eval_call(context: &EvaluationContext, call: &Call, input: Value) -> Result<V
                 },
             )
         }
+
+        for named in decl.signature().named {
+            for call_named in &call.named {
+                if call_named.0.item == named.long {
+                    let var_id = named
+                        .var_id
+                        .expect("internal error: all custom parameters must have var_ids");
+                    if let Some(arg) = &call_named.1 {
+                        let result = eval_expression(&state, arg)?;
+
+                        state.add_var(var_id, result);
+                    } else {
+                        state.add_var(
+                            var_id,
+                            Value::Bool {
+                                val: true,
+                                span: call.head,
+                            },
+                        )
+                    }
+                }
+            }
+        }
         let engine_state = state.engine_state.borrow();
         let block = engine_state.get_block(block_id);
         eval_block(&state, block, input)
@@ -98,7 +121,13 @@ fn eval_external(
     }
 
     if last_expression {
-        call.named.push(("last_expression".into(), None))
+        call.named.push((
+            Spanned {
+                item: "last_expression".into(),
+                span: Span::unknown(),
+            },
+            None,
+        ))
     }
 
     command.run(context, &call, input)
diff --git a/crates/nu-parser/src/flatten.rs b/crates/nu-parser/src/flatten.rs
index db94d622b3..5bb3a2103a 100644
--- a/crates/nu-parser/src/flatten.rs
+++ b/crates/nu-parser/src/flatten.rs
@@ -18,6 +18,7 @@ pub enum FlatShape {
     Filepath,
     GlobPattern,
     Variable,
+    Flag,
     Custom(String),
 }
 
@@ -61,6 +62,12 @@ pub fn flatten_expression(
             for positional in &call.positional {
                 output.extend(flatten_expression(working_set, positional));
             }
+            for named in &call.named {
+                output.push((named.0.span, FlatShape::Flag));
+                if let Some(expr) = &named.1 {
+                    output.extend(flatten_expression(working_set, expr));
+                }
+            }
             output
         }
         Expr::ExternalCall(_, name_span, args) => {
diff --git a/crates/nu-parser/src/parser.rs b/crates/nu-parser/src/parser.rs
index e1cdcb9781..da71974f05 100644
--- a/crates/nu-parser/src/parser.rs
+++ b/crates/nu-parser/src/parser.rs
@@ -57,7 +57,7 @@ fn check_call(command: Span, sig: &Signature, call: &Call) -> Option<ParseError>
         Some(ParseError::MissingPositional(missing.name.clone(), command))
     } else {
         for req_flag in sig.named.iter().filter(|x| x.required) {
-            if call.named.iter().all(|(n, _)| n != &req_flag.long) {
+            if call.named.iter().all(|(n, _)| n.item != req_flag.long) {
                 return Some(ParseError::MissingRequiredFlag(
                     req_flag.long.clone(),
                     command,
@@ -478,7 +478,13 @@ pub fn parse_internal_call(
         if let Some(long_name) = long_name {
             // We found a long flag, like --bar
             error = error.or(err);
-            call.named.push((long_name, arg));
+            call.named.push((
+                Spanned {
+                    item: long_name,
+                    span: arg_span,
+                },
+                arg,
+            ));
             spans_idx += 1;
             continue;
         }
@@ -500,13 +506,25 @@ pub fn parse_internal_call(
                         let (arg, err) = parse_value(working_set, *arg, &arg_shape);
                         error = error.or(err);
 
-                        call.named.push((flag.long.clone(), Some(arg)));
+                        call.named.push((
+                            Spanned {
+                                item: flag.long.clone(),
+                                span: spans[spans_idx],
+                            },
+                            Some(arg),
+                        ));
                         spans_idx += 1;
                     } else {
                         error = error.or(Some(ParseError::MissingFlagParam(arg_span)))
                     }
                 } else {
-                    call.named.push((flag.long.clone(), None));
+                    call.named.push((
+                        Spanned {
+                            item: flag.long.clone(),
+                            span: spans[spans_idx],
+                        },
+                        None,
+                    ));
                 }
             }
             spans_idx += 1;
@@ -1850,7 +1868,18 @@ pub fn parse_signature(
 
     if bytes.starts_with(b"[") {
         start += 1;
+    } else {
+        error = error.or_else(|| {
+            Some(ParseError::Expected(
+                "[".into(),
+                Span {
+                    start,
+                    end: start + 1,
+                },
+            ))
+        });
     }
+
     if bytes.ends_with(b"]") {
         end -= 1;
     } else {
diff --git a/crates/nu-protocol/src/ast/call.rs b/crates/nu-protocol/src/ast/call.rs
index 1f2a4870e7..815e18a649 100644
--- a/crates/nu-protocol/src/ast/call.rs
+++ b/crates/nu-protocol/src/ast/call.rs
@@ -1,5 +1,5 @@
 use super::Expression;
-use crate::{DeclId, Span};
+use crate::{DeclId, Span, Spanned};
 
 #[derive(Debug, Clone)]
 pub struct Call {
@@ -7,7 +7,7 @@ pub struct Call {
     pub decl_id: DeclId,
     pub head: Span,
     pub positional: Vec<Expression>,
-    pub named: Vec<(String, Option<Expression>)>,
+    pub named: Vec<(Spanned<String>, Option<Expression>)>,
 }
 
 impl Default for Call {
@@ -28,7 +28,7 @@ impl Call {
 
     pub fn has_flag(&self, flag_name: &str) -> bool {
         for name in &self.named {
-            if flag_name == name.0 {
+            if flag_name == name.0.item {
                 return true;
             }
         }
@@ -38,7 +38,7 @@ impl Call {
 
     pub fn get_flag_expr(&self, flag_name: &str) -> Option<Expression> {
         for name in &self.named {
-            if flag_name == name.0 {
+            if flag_name == name.0.item {
                 return name.1.clone();
             }
         }
diff --git a/src/tests.rs b/src/tests.rs
index 1d441cc3ee..fbfbf67d99 100644
--- a/src/tests.rs
+++ b/src/tests.rs
@@ -709,3 +709,16 @@ fn missing_column_error() -> TestResult {
         "cannot find column",
     )
 }
+
+#[test]
+fn missing_parameters() -> TestResult {
+    fail_test(r#"def foo {}"#, "expected [")
+}
+
+#[test]
+fn flag_param_value() -> TestResult {
+    run_test(
+        r#"def foo [--bob: int] { $bob + 100 }; foo --bob 55"#,
+        "155",
+    )
+}