diff --git a/frame/support/src/traits.rs b/frame/support/src/traits.rs
index b36559c3637dc..f7e7710b32936 100644
--- a/frame/support/src/traits.rs
+++ b/frame/support/src/traits.rs
@@ -1431,11 +1431,19 @@ impl<BlockNumber: Clone> OnInitialize<BlockNumber> for Tuple {
 	}
 }
 
-/// The runtime upgrade trait. Implementing this lets you express what should happen
-/// when the runtime upgrades, and changes may need to occur to your module.
+/// The runtime upgrade trait.
+///
+/// Implementing this lets you express what should happen when the runtime upgrades,
+/// and changes may need to occur to your module.
 pub trait OnRuntimeUpgrade {
 	/// Perform a module upgrade.
 	///
+	/// # Warning
+	///
+	/// This function will be called before we initialized any runtime state, aka `on_initialize`
+	/// wasn't called yet. So, information like the block number and any other
+	/// block local data are not accessible.
+	///
 	/// Return the non-negotiable weight consumed for runtime upgrade.
 	fn on_runtime_upgrade() -> crate::weights::Weight { 0 }
 }