diff --git a/crates/nu-command/tests/commands/def.rs b/crates/nu-command/tests/commands/def.rs
index c39e89ada2..0ec13c0511 100644
--- a/crates/nu-command/tests/commands/def.rs
+++ b/crates/nu-command/tests/commands/def.rs
@@ -172,19 +172,6 @@ fn def_default_value_should_restrict_implicit_type() {
     assert!(actual2.err.contains("expected int"));
 }
 
-#[test]
-fn def_boolean_flags() {
-    let actual = nu!("def foo [--x: bool] { $x }; foo --x");
-    assert!(actual.err.contains("flag missing bool argument"));
-    let actual = nu!("def foo [--x: bool = false] { $x }; foo");
-    assert_eq!(actual.out, "false");
-    let actual = nu!("def foo [--x: bool = false] { $x }; foo --x");
-    assert!(actual.err.contains("flag missing bool argument"));
-    // boolean flags' default value should be null
-    let actual = nu!("def foo [--x: bool] { $x == null }; foo");
-    assert_eq!(actual.out, "true");
-}
-
 #[test]
 fn def_wrapped_with_block() {
     let actual = nu!(
diff --git a/crates/nu-parser/src/parser.rs b/crates/nu-parser/src/parser.rs
index 6e1ab7fc6f..a89b0dfdaa 100644
--- a/crates/nu-parser/src/parser.rs
+++ b/crates/nu-parser/src/parser.rs
@@ -18,8 +18,8 @@ use nu_protocol::{
     },
     engine::StateWorkingSet,
     eval_const::eval_constant,
-    span, BlockId, DidYouMean, Flag, ParseError, ParseWarning, PositionalArg, Signature, Span,
-    Spanned, SyntaxShape, Type, Unit, VarId, ENV_VARIABLE_ID, IN_VARIABLE_ID,
+    span, BlockId, DidYouMean, Flag, ParseError, PositionalArg, Signature, Span, Spanned,
+    SyntaxShape, Type, Unit, VarId, ENV_VARIABLE_ID, IN_VARIABLE_ID,
 };
 
 use crate::parse_keywords::{
@@ -3603,9 +3603,9 @@ pub fn parse_signature_helper(working_set: &mut StateWorkingSet, span: Span) ->
                                     } => {
                                         working_set.set_variable_type(var_id.expect("internal error: all custom parameters must have var_ids"), syntax_shape.to_type());
                                         if syntax_shape == SyntaxShape::Boolean {
-                                            working_set.warning(ParseWarning::DeprecatedWarning(
-                                                "--flag: bool".to_string(),
-                                                "--flag".to_string(),
+                                            working_set.error(ParseError::LabeledError(
+                                                "Type annotations are not allowed for boolean switches.".to_string(),
+                                                "Remove the `: bool` type annotation.".to_string(),
                                                 span,
                                             ));
                                         }
diff --git a/src/tests/test_custom_commands.rs b/src/tests/test_custom_commands.rs
index 48e575a14b..fe6f8dfa76 100644
--- a/src/tests/test_custom_commands.rs
+++ b/src/tests/test_custom_commands.rs
@@ -149,8 +149,7 @@ fn custom_flag2() -> TestResult {
 #[test]
 fn deprecated_boolean_flag() {
     let actual = nu!(r#"def florb [--dry-run: bool, --another-flag] { "aaa" };  florb"#);
-    assert!(actual.err.contains("Deprecated"));
-    assert_eq!(actual.out, "aaa");
+    assert!(actual.err.contains("not allowed"));
 }
 
 #[test]