diff --git a/cdk/wo_stack.py b/cdk/wo_stack.py
index def3964..05d8ac2 100644
--- a/cdk/wo_stack.py
+++ b/cdk/wo_stack.py
@@ -418,8 +418,12 @@ class WorkorderIngestStack(Stack):
         )
         fallback_rate = cloudwatch.MathExpression(
             expression=(
+                # The IF volume floor (>=10) already guarantees the denominator
+                # is non-zero in the true branch, so divide directly. (An earlier
+                # MAX([...,1]) divide-by-zero guard used array syntax CloudWatch
+                # rejects at deploy: "Unsupported operand type(s) for MAX".)
                 "IF((FILL(fb,0)+FILL(tmpl,0))>=10, "
-                "100*FILL(fb,0)/MAX([FILL(fb,0)+FILL(tmpl,0),1]), 0)"
+                "100*FILL(fb,0)/(FILL(fb,0)+FILL(tmpl,0)), 0)"
             ),
             using_metrics={"fb": fb_metric, "tmpl": tmpl_metric},
             period=Duration.minutes(15),