diff --git a/src/promql/src/functions/extrapolate_rate.rs b/src/promql/src/functions/extrapolate_rate.rs
index cc70448d20..a084fe480f 100644
--- a/src/promql/src/functions/extrapolate_rate.rs
+++ b/src/promql/src/functions/extrapolate_rate.rs
@@ -69,7 +69,9 @@ impl<const IS_COUNTER: bool, const IS_RATE: bool> ExtrapolatedRate<IS_COUNTER, I
             (true, true) => "prom_rate",
             (true, false) => "prom_increase",
             (false, false) => "prom_delta",
-            (false, true) => "prom_delta",
+            (false, true) => {
+                unreachable!("gauge rate is not supported by ExtrapolatedRate")
+            }
         }
     }