diff --git a/common/decls/decls.go b/common/decls/decls.go
index 0a42f81d..1bf4667e 100644
--- a/common/decls/decls.go
+++ b/common/decls/decls.go
@@ -251,15 +251,15 @@ func (f *FunctionDecl) Bindings() ([]*functions.Overload, error) {
 			// are preserved in order to assist with the function resolution step.
 			switch len(args) {
 			case 1:
-				if o.unaryOp != nil && o.matchesRuntimeSignature( /* disableTypeGuards=*/ false, args...) {
+				if o.unaryOp != nil && o.matchesRuntimeSignature(f.disableTypeGuards, args...) {
 					return o.unaryOp(args[0])
 				}
 			case 2:
-				if o.binaryOp != nil && o.matchesRuntimeSignature( /* disableTypeGuards=*/ false, args...) {
+				if o.binaryOp != nil && o.matchesRuntimeSignature(f.disableTypeGuards, args...) {
 					return o.binaryOp(args[0], args[1])
 				}
 			}
-			if o.functionOp != nil && o.matchesRuntimeSignature( /* disableTypeGuards=*/ false, args...) {
+			if o.functionOp != nil && o.matchesRuntimeSignature(f.disableTypeGuards, args...) {
 				return o.functionOp(args...)
 			}
 			// eventually this will fall through to the noSuchOverload below.
diff --git a/ext/BUILD.bazel b/ext/BUILD.bazel
index db223da2..3fe5378f 100644
--- a/ext/BUILD.bazel
+++ b/ext/BUILD.bazel
@@ -24,6 +24,7 @@ go_library(
         "//cel:go_default_library",
         "//checker:go_default_library",
         "//common/ast:go_default_library",
+        "//common/decls:go_default_library",
         "//common/overloads:go_default_library",
         "//common/operators:go_default_library",
         "//common/types:go_default_library",
@@ -61,8 +62,8 @@ go_test(
         "//common/types/ref:go_default_library",
         "//common/types/traits:go_default_library",
         "//test:go_default_library",
-        "//test/proto2pb:go_default_library",   
-        "//test/proto3pb:go_default_library",     
+        "//test/proto2pb:go_default_library",
+        "//test/proto3pb:go_default_library",
         "@org_golang_google_protobuf//proto:go_default_library",
         "@org_golang_google_protobuf//types/known/wrapperspb:go_default_library",
         "@org_golang_google_protobuf//encoding/protojson:go_default_library",
diff --git a/ext/lists.go b/ext/lists.go
index de2fd709..cfce9a72 100644
--- a/ext/lists.go
+++ b/ext/lists.go
@@ -19,6 +19,7 @@ import (
 	"math"
 
 	"github.com/google/cel-go/cel"
+	"github.com/google/cel-go/common/decls"
 	"github.com/google/cel-go/common/types"
 	"github.com/google/cel-go/common/types/ref"
 	"github.com/google/cel-go/common/types/traits"
@@ -95,6 +96,7 @@ func ListsVersion(version uint32) ListsOption {
 // CompileOptions implements the Library interface method.
 func (lib listsLib) CompileOptions() []cel.EnvOption {
 	listType := cel.ListType(cel.TypeParamType("T"))
+	listListType := cel.ListType(listType)
 	listDyn := cel.ListType(cel.DynType)
 	opts := []cel.EnvOption{
 		cel.Function("slice",
@@ -117,24 +119,35 @@ func (lib listsLib) CompileOptions() []cel.EnvOption {
 		opts = append(opts,
 			cel.Function("flatten",
 				cel.MemberOverload("list_flatten",
-					[]*cel.Type{listDyn}, listDyn,
+					[]*cel.Type{listListType}, listType,
 					cel.UnaryBinding(func(arg ref.Val) ref.Val {
-						list := arg.(traits.Lister)
+						list, ok := arg.(traits.Lister)
+						if !ok {
+							return types.MaybeNoSuchOverloadErr(arg)
+						}
 						flatList := flatten(list, 1)
 						return types.DefaultTypeAdapter.NativeToValue(flatList)
 					}),
 				),
-			),
-			cel.Function("flatten",
 				cel.MemberOverload("list_flatten_int",
 					[]*cel.Type{listDyn, types.IntType}, listDyn,
 					cel.BinaryBinding(func(arg1, arg2 ref.Val) ref.Val {
-						list := arg1.(traits.Lister)
-						depth := arg2.(types.Int)
+						list, ok := arg1.(traits.Lister)
+						if !ok {
+							return types.MaybeNoSuchOverloadErr(arg1)
+						}
+						depth, ok := arg2.(types.Int)
+						if !ok {
+							return types.MaybeNoSuchOverloadErr(arg2)
+						}
 						flatList := flatten(list, int64(depth))
 						return types.DefaultTypeAdapter.NativeToValue(flatList)
 					}),
 				),
+				// To handle the case where a variable of just `list(T)` is provided at runtime
+				// with a graceful failure more, disable the type guards since the implementation
+				// can handle lists which are already flat.
+				decls.DisableTypeGuards(true),
 			),
 		)
 	}
diff --git a/ext/lists_test.go b/ext/lists_test.go
index 1851f838..beeb0ee7 100644
--- a/ext/lists_test.go
+++ b/ext/lists_test.go
@@ -36,8 +36,8 @@ func TestLists(t *testing.T) {
 		{expr: `[1,2,3,4].slice(0, 10)`, err: "cannot slice(0, 10), list is length 4"},
 		{expr: `[1,2,3,4].slice(-5, 10)`, err: "cannot slice(-5, 10), negative indexes not supported"},
 		{expr: `[1,2,3,4].slice(-5, -3)`, err: "cannot slice(-5, -3), negative indexes not supported"},
-		{expr: `[].flatten() == []`},
-		{expr: `[1,2,3,4].flatten() == [1,2,3,4]`},
+		{expr: `dyn([]).flatten() == []`},
+		{expr: `dyn([1,2,3,4]).flatten() == [1,2,3,4]`},
 		{expr: `[1,[2,[3,4]]].flatten() == [1,2,[3,4]]`},
 		{expr: `[1,2,[],[],[3,4]].flatten() == [1,2,3,4]`},
 		{expr: `[1,[2,[3,4]]].flatten(2) == [1,2,3,4]`},