diff --git a/crates/nu-command/tests/commands/alias.rs b/crates/nu-command/tests/commands/alias.rs
index 3aa9e024c..3600e83e4 100644
--- a/crates/nu-command/tests/commands/alias.rs
+++ b/crates/nu-command/tests/commands/alias.rs
@@ -51,7 +51,8 @@ fn alias_fails_with_invalid_name() {
     ));
     assert!(actual
         .err
-        .contains("alias name can't be a number or a filesize"));
+        .contains("alias name can't be a number, a filesize, or contain a hash"));
+
     let actual = nu!(
         cwd: "tests/fixtures/formats", pipeline(
         r#"
@@ -60,7 +61,17 @@ fn alias_fails_with_invalid_name() {
     ));
     assert!(actual
         .err
-        .contains("alias name can't be a number or a filesize"));
+        .contains("alias name can't be a number, a filesize, or contain a hash"));
+
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            alias "te#t" = echo "test"   
+        "#
+    ));
+    assert!(actual
+        .err
+        .contains("alias name can't be a number, a filesize, or contain a hash"));
 }
 
 #[test]
diff --git a/crates/nu-command/tests/commands/def.rs b/crates/nu-command/tests/commands/def.rs
index fdd2d0cb3..8323e275a 100644
--- a/crates/nu-command/tests/commands/def.rs
+++ b/crates/nu-command/tests/commands/def.rs
@@ -30,3 +30,36 @@ fn def_errors_with_multiple_short_flags() {
 
     assert!(actual.err.contains("expected one short flag"));
 }
+
+#[test]
+fn def_fails_with_invalid_name() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            def 1234 = echo "test"
+        "#
+    ));
+    assert!(actual
+        .err
+        .contains("command name can't be a number, a filesize, or contain a hash"));
+
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            def 5gib = echo "test"
+        "#
+    ));
+    assert!(actual
+        .err
+        .contains("command name can't be a number, a filesize, or contain a hash"));
+
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            def "te#t" = echo "test"
+        "#
+    ));
+    assert!(actual
+        .err
+        .contains("command name can't be a number, a filesize, or contain a hash"));
+}
diff --git a/crates/nu-parser/src/errors.rs b/crates/nu-parser/src/errors.rs
index d9ea6565b..c48398f9b 100644
--- a/crates/nu-parser/src/errors.rs
+++ b/crates/nu-parser/src/errors.rs
@@ -172,7 +172,13 @@ pub enum ParseError {
 
     #[error("Alias name not supported.")]
     #[diagnostic(code(nu::parser::variable_not_valid), url(docsrs))]
-    AliasNotValid(#[label = "alias name can't be a number or a filesize"] Span),
+    AliasNotValid(#[label = "alias name can't be a number, a filesize, or contain a hash"] Span),
+
+    #[error("Command name not supported.")]
+    #[diagnostic(code(nu::parser::variable_not_valid), url(docsrs))]
+    CommandDefNotValid(
+        #[label = "command name can't be a number, a filesize, or contain a hash"] Span,
+    ),
 
     #[error("Module not found.")]
     #[diagnostic(
@@ -419,6 +425,7 @@ impl ParseError {
             ParseError::VariableNotFound(s) => *s,
             ParseError::VariableNotValid(s) => *s,
             ParseError::AliasNotValid(s) => *s,
+            ParseError::CommandDefNotValid(s) => *s,
             ParseError::ModuleNotFound(s) => *s,
             ParseError::CyclicalModuleImport(_, s) => *s,
             ParseError::ModuleOrOverlayNotFound(s) => *s,
diff --git a/crates/nu-parser/src/parse_keywords.rs b/crates/nu-parser/src/parse_keywords.rs
index 5705c389b..da60abd5a 100644
--- a/crates/nu-parser/src/parse_keywords.rs
+++ b/crates/nu-parser/src/parse_keywords.rs
@@ -57,6 +57,13 @@ pub fn parse_def_predecl(
         let signature = sig.as_signature();
         working_set.exit_scope();
         if let (Some(name), Some(mut signature)) = (name, signature) {
+            if name.contains('#')
+                || name.parse::<bytesize::ByteSize>().is_ok()
+                || name.parse::<f64>().is_ok()
+            {
+                return Some(ParseError::CommandDefNotValid(spans[1]));
+            }
+
             signature.name = name;
             let decl = signature.predeclare();
 
@@ -80,6 +87,13 @@ pub fn parse_def_predecl(
         working_set.exit_scope();
 
         if let (Some(name), Some(mut signature)) = (name, signature) {
+            if name.contains('#')
+                || name.parse::<bytesize::ByteSize>().is_ok()
+                || name.parse::<f64>().is_ok()
+            {
+                return Some(ParseError::CommandDefNotValid(spans[1]));
+            }
+
             signature.name = name.clone();
             //let decl = signature.predeclare();
             let decl = KnownExternal {
@@ -593,27 +607,16 @@ pub fn parse_alias(
         return (Pipeline::from_vec(vec![expr]), None);
     }
 
-    let (name_span, alias_name, split_id) =
+    let (name_span, split_id) =
         if spans.len() > 1 && working_set.get_span_contents(spans[0]) == b"export" {
-            (spans[1], spans.get(2), 2)
+            (spans[1], 2)
         } else {
-            (spans[0], spans.get(1), 1)
+            (spans[0], 1)
         };
 
     let name = working_set.get_span_contents(name_span);
 
     if name == b"alias" {
-        if let Some(alias_name) = alias_name {
-            let alias_name = String::from_utf8_lossy(working_set.get_span_contents(*alias_name));
-            if alias_name.parse::<bytesize::ByteSize>().is_ok() || alias_name.parse::<f64>().is_ok()
-            {
-                return (
-                    Pipeline::from_vec(vec![garbage(name_span)]),
-                    Some(ParseError::AliasNotValid(name_span)),
-                );
-            }
-        }
-
         if let Some((span, err)) = check_name(working_set, spans) {
             return (Pipeline::from_vec(vec![garbage(*span)]), Some(err));
         }
@@ -656,6 +659,17 @@ pub fn parse_alias(
 
                 let replacement = spans[(split_id + 2)..].to_vec();
 
+                let checked_name = String::from_utf8_lossy(&alias_name);
+                if checked_name.contains('#')
+                    || checked_name.parse::<bytesize::ByteSize>().is_ok()
+                    || checked_name.parse::<f64>().is_ok()
+                {
+                    return (
+                        Pipeline::from_vec(vec![garbage(name_span)]),
+                        Some(ParseError::AliasNotValid(name_span)),
+                    );
+                }
+
                 working_set.add_alias(alias_name, replacement);
             }