diff --git a/crates/typst-eval/src/vm.rs b/crates/typst-eval/src/vm.rs
index 1c8331b66..adf7dd762 100644
--- a/crates/typst-eval/src/vm.rs
+++ b/crates/typst-eval/src/vm.rs
@@ -1,4 +1,5 @@
 use comemo::Tracked;
+use typst_library::diag::warning;
 use typst_library::engine::Engine;
 use typst_library::foundations::{Context, IntoValue, Scopes, Value};
 use typst_library::World;
@@ -47,6 +48,16 @@ impl<'a> Vm<'a> {
         if self.inspected == Some(var.span()) {
             self.trace(value.clone());
         }
+        // This will become an error in the parser if 'is' becomes a keyword.
+        if var.get() == "is" {
+            self.engine.sink.warn(warning!(
+                var.span(),
+                "`is` will likely become a keyword in future versions and will \
+                not be allowed as an identifier";
+                hint: "rename this variable to avoid future errors";
+                hint: "try `is_` instead"
+            ));
+        }
         self.scopes.top.define_ident(var, value);
     }