diff --git a/utils.go b/utils.go
index c146981..ee0b211 100644
--- a/utils.go
+++ b/utils.go
@@ -84,7 +84,7 @@ func IsLikelyInvalid(e Expr) bool {
 			return
 		}
 		idx := GetRollupArgIdx(fe)
-		if idx < 0 {
+		if idx < 0 || idx >= len(fe.Args) {
 			return
 		}
 		arg := fe.Args[idx]
diff --git a/utils_test.go b/utils_test.go
index b60ba3d..6064820 100644
--- a/utils_test.go
+++ b/utils_test.go
@@ -117,6 +117,9 @@ func TestIsLikelyInvalid(t *testing.T) {
 	f(`rate(rate(foo))`, true)
 	f(`1 + rate(label_set(foo, "bar", "baz"))`, true)
 	f(`rate(sum(foo) offset 5m)`, true)
+
+	// invalid number of args
+	f(`quantile_over_time(foo)`, false)
 }
 
 func TestIsSupportedFunction(t *testing.T) {