diff --git a/crates/typst-syntax/src/lexer.rs b/crates/typst-syntax/src/lexer.rs
index 1314016fa..358c25b20 100644
--- a/crates/typst-syntax/src/lexer.rs
+++ b/crates/typst-syntax/src/lexer.rs
@@ -766,6 +766,12 @@ impl Lexer<'_> {
             return self.error(eco_format!("invalid number suffix: {}", suffix));
         }
 
+        if base != 10 {
+            let kind = self.error(eco_format!("invalid base-{base} prefix"));
+            self.hint("numbers with a unit cannot have a base prefix");
+            return kind;
+        }
+
         SyntaxKind::Numeric
     }
 
diff --git a/tests/suite/layout/length.typ b/tests/suite/layout/length.typ
index 5ba70072d..71d79da9a 100644
--- a/tests/suite/layout/length.typ
+++ b/tests/suite/layout/length.typ
@@ -74,3 +74,8 @@
 // Hint: 2-24 use `length.to-absolute()` to resolve its em component (requires context)
 // Hint: 2-24 or use `length.abs.inches()` instead to ignore its em component
 #(4.5em + 6in).inches()
+
+--- issue-5519-length-base ---
+// Error: 2-9 invalid base-2 prefix
+// Hint: 2-9 numbers with a unit cannot have a base prefix
+#0b100pt