diff --git a/cel/cel_test.go b/cel/cel_test.go
index e80a3288..b27bd6d1 100644
--- a/cel/cel_test.go
+++ b/cel/cel_test.go
@@ -1195,6 +1195,14 @@ func TestResidualAstMacros(t *testing.T) {
 	}
 }
 
+func TestResidualAstNil(t *testing.T) {
+	env := testEnv(t)
+	ast, err := env.ResidualAst(nil, nil)
+	if err == nil || !strings.Contains(err.Error(), "unsupported expr") {
+		t.Errorf("env.ResidualAst() got (%v, %v) wanted unsupported expr error", ast, err)
+	}
+}
+
 func BenchmarkEvalOptions(b *testing.B) {
 	env := testEnv(b,
 		Variable("ai", IntType),
@@ -1323,7 +1331,7 @@ func TestEnvExtensionIsolation(t *testing.T) {
 func TestVariadicLogicalOperators(t *testing.T) {
 	env := testEnv(t, variadicLogicalOperatorASTs())
 	ast, iss := env.Compile(
-		`(false || false || false || false || true) && 
+		`(false || false || false || false || true) &&
 		 (true && true && true && true && false)`)
 	if iss.Err() != nil {
 		t.Fatalf("Compile() failed: %v", iss.Err())
@@ -2293,7 +2301,7 @@ func TestOptionalValuesCompile(t *testing.T) {
 			if iss.Err() != nil {
 				t.Fatalf("%v failed: %v", tc.expr, iss.Err())
 			}
-			for id, reference := range ast.impl.ReferenceMap() {
+			for id, reference := range ast.NativeRep().ReferenceMap() {
 				other, found := tc.references[id]
 				if !found {
 					t.Errorf("Compile(%v) expected reference %d: %v", tc.expr, id, reference)
@@ -2955,6 +2963,15 @@ func BenchmarkDynamicDispatch(b *testing.B) {
 	})
 }
 
+func TestAstProgramNilValue(t *testing.T) {
+	var ast *Ast = nil
+	env := testEnv(t)
+	prg, err := env.Program(ast)
+	if err == nil || !strings.Contains(err.Error(), "unsupported expr") {
+		t.Errorf("env.Program() got (%v,%v) wanted unsupported expr error", prg, err)
+	}
+}
+
 // TODO: ideally testCostEstimator and testRuntimeCostEstimator would be shared in a test fixtures package
 type testCostEstimator struct {
 	hints map[string]uint64
diff --git a/cel/env.go b/cel/env.go
index ab736b77..caee8e8c 100644
--- a/cel/env.go
+++ b/cel/env.go
@@ -556,7 +556,8 @@ func (e *Env) PartialVars(vars any) (interpreter.PartialActivation, error) {
 // TODO: Consider adding an option to generate a Program.Residual to avoid round-tripping to an
 // Ast format and then Program again.
 func (e *Env) ResidualAst(a *Ast, details *EvalDetails) (*Ast, error) {
-	pruned := interpreter.PruneAst(a.impl.Expr(), a.impl.SourceInfo().MacroCalls(), details.State())
+	ast := a.NativeRep()
+	pruned := interpreter.PruneAst(ast.Expr(), ast.SourceInfo().MacroCalls(), details.State())
 	newAST := &Ast{source: a.Source(), impl: pruned}
 	expr, err := AstToString(newAST)
 	if err != nil {
@@ -582,7 +583,7 @@ func (e *Env) EstimateCost(ast *Ast, estimator checker.CostEstimator, opts ...ch
 	extendedOpts := make([]checker.CostOption, 0, len(e.costOptions))
 	extendedOpts = append(extendedOpts, opts...)
 	extendedOpts = append(extendedOpts, e.costOptions...)
-	return checker.Cost(ast.impl, estimator, extendedOpts...)
+	return checker.Cost(ast.NativeRep(), estimator, extendedOpts...)
 }
 
 // configure applies a series of EnvOptions to the current environment.
diff --git a/cel/folding_test.go b/cel/folding_test.go
index 52357cc3..3f24f50e 100644
--- a/cel/folding_test.go
+++ b/cel/folding_test.go
@@ -650,11 +650,11 @@ func TestConstantFoldingNormalizeIDs(t *testing.T) {
 				t.Fatalf("Compile() failed: %v", iss.Err())
 			}
 			preOpt := newIDCollector()
-			ast.PostOrderVisit(checked.impl.Expr(), preOpt)
+			ast.PostOrderVisit(checked.NativeRep().Expr(), preOpt)
 			if !reflect.DeepEqual(preOpt.IDs(), tc.ids) {
 				t.Errorf("Compile() got ids %v, expected %v", preOpt.IDs(), tc.ids)
 			}
-			for id, call := range checked.impl.SourceInfo().MacroCalls() {
+			for id, call := range checked.NativeRep().SourceInfo().MacroCalls() {
 				macroText, found := tc.macros[id]
 				if !found {
 					t.Fatalf("Compile() did not find macro %d", id)
@@ -682,11 +682,11 @@ func TestConstantFoldingNormalizeIDs(t *testing.T) {
 				t.Fatalf("Optimize() generated an invalid AST: %v", iss.Err())
 			}
 			postOpt := newIDCollector()
-			ast.PostOrderVisit(optimized.impl.Expr(), postOpt)
+			ast.PostOrderVisit(optimized.NativeRep().Expr(), postOpt)
 			if !reflect.DeepEqual(postOpt.IDs(), tc.normalizedIDs) {
 				t.Errorf("Optimize() got ids %v, expected %v", postOpt.IDs(), tc.normalizedIDs)
 			}
-			for id, call := range optimized.impl.SourceInfo().MacroCalls() {
+			for id, call := range optimized.NativeRep().SourceInfo().MacroCalls() {
 				macroText, found := tc.normalizedMacros[id]
 				if !found {
 					t.Fatalf("Optimize() did not find macro %d", id)
diff --git a/cel/inlining.go b/cel/inlining.go
index 78d5bea6..a4530e19 100644
--- a/cel/inlining.go
+++ b/cel/inlining.go
@@ -60,7 +60,7 @@ func NewInlineVariable(name string, definition *Ast) *InlineVariable {
 // If the variable occurs more than once, the provided alias will be used to replace the expressions
 // where the variable name occurs.
 func NewInlineVariableWithAlias(name, alias string, definition *Ast) *InlineVariable {
-	return &InlineVariable{name: name, alias: alias, def: definition.impl}
+	return &InlineVariable{name: name, alias: alias, def: definition.NativeRep()}
 }
 
 // NewInliningOptimizer creates and optimizer which replaces variables with expression definitions.
diff --git a/cel/io.go b/cel/io.go
index dd010a99..a327c967 100644
--- a/cel/io.go
+++ b/cel/io.go
@@ -62,7 +62,7 @@ func AstToCheckedExpr(a *Ast) (*exprpb.CheckedExpr, error) {
 	if !a.IsChecked() {
 		return nil, fmt.Errorf("cannot convert unchecked ast")
 	}
-	return ast.ToProto(a.impl)
+	return ast.ToProto(a.NativeRep())
 }
 
 // ParsedExprToAst converts a parsed expression proto message to an Ast.
@@ -99,7 +99,7 @@ func AstToParsedExpr(a *Ast) (*exprpb.ParsedExpr, error) {
 // Note, the conversion may not be an exact replica of the original expression, but will produce
 // a string that is semantically equivalent and whose textual representation is stable.
 func AstToString(a *Ast) (string, error) {
-	return parser.Unparse(a.impl.Expr(), a.impl.SourceInfo())
+	return parser.Unparse(a.NativeRep().Expr(), a.NativeRep().SourceInfo())
 }
 
 // RefValueToValue converts between ref.Val and google.api.expr.v1alpha1.Value.
diff --git a/cel/io_test.go b/cel/io_test.go
index d1710f4d..7bc34ee9 100644
--- a/cel/io_test.go
+++ b/cel/io_test.go
@@ -16,6 +16,7 @@ package cel
 
 import (
 	"fmt"
+	"strings"
 	"testing"
 	"time"
 
@@ -144,6 +145,27 @@ func TestAstToString(t *testing.T) {
 	}
 }
 
+func TestAstToStringNil(t *testing.T) {
+	expr, err := AstToString(nil)
+	if err == nil || !strings.Contains(err.Error(), "unsupported expr") {
+		t.Errorf("env.AstToString() got (%v, %v) wanted unsupported expr error", expr, err)
+	}
+}
+
+func TestAstToCheckedExprNil(t *testing.T) {
+	expr, err := AstToCheckedExpr(nil)
+	if err == nil || !strings.Contains(err.Error(), "cannot convert unchecked ast") {
+		t.Errorf("env.AstToCheckedExpr() got (%v, %v) wanted conversion error", expr, err)
+	}
+}
+
+func TestAstToParsedExprNil(t *testing.T) {
+	expr, err := AstToParsedExpr(nil)
+	if err != nil {
+		t.Errorf("env.AstToParsedExpr() got (%v, %v) wanted conversion error", expr, err)
+	}
+}
+
 func TestCheckedExprToAstConstantExpr(t *testing.T) {
 	stdEnv, err := NewEnv()
 	if err != nil {
diff --git a/cel/optimizer.go b/cel/optimizer.go
index c149abb7..9a2a97a6 100644
--- a/cel/optimizer.go
+++ b/cel/optimizer.go
@@ -48,8 +48,8 @@ func NewStaticOptimizer(optimizers ...ASTOptimizer) *StaticOptimizer {
 // If issues are encountered, the Issues.Err() return value will be non-nil.
 func (opt *StaticOptimizer) Optimize(env *Env, a *Ast) (*Ast, *Issues) {
 	// Make a copy of the AST to be optimized.
-	optimized := ast.Copy(a.impl)
-	ids := newIDGenerator(ast.MaxID(a.impl))
+	optimized := ast.Copy(a.NativeRep())
+	ids := newIDGenerator(ast.MaxID(a.NativeRep()))
 
 	// Create the optimizer context, could be pooled in the future.
 	issues := NewIssues(common.NewErrors(a.Source()))
@@ -86,7 +86,7 @@ func (opt *StaticOptimizer) Optimize(env *Env, a *Ast) (*Ast, *Issues) {
 		if iss.Err() != nil {
 			return nil, iss
 		}
-		optimized = checked.impl
+		optimized = checked.NativeRep()
 	}
 
 	// Return the optimized result.
diff --git a/cel/optimizer_test.go b/cel/optimizer_test.go
index 2faac17e..f406e7c7 100644
--- a/cel/optimizer_test.go
+++ b/cel/optimizer_test.go
@@ -16,6 +16,7 @@ package cel_test
 
 import (
 	"sort"
+	"strings"
 	"testing"
 
 	"github.com/google/cel-go/cel"
@@ -201,6 +202,15 @@ func TestStaticOptimizerNewAST(t *testing.T) {
 	}
 }
 
+func TestStaticOptimizerNilAST(t *testing.T) {
+	env := optimizerEnv(t)
+	opt := cel.NewStaticOptimizer(&identityOptimizer{t: t})
+	optAST, iss := opt.Optimize(env, nil)
+	if iss.Err() == nil || !strings.Contains(iss.Err().Error(), "unexpected unspecified type") {
+		t.Errorf("opt.Optimize(env, nil) got (%v, %v), wanted unexpected unspecified type", optAST, iss)
+	}
+}
+
 type identityOptimizer struct {
 	t *testing.T
 }
diff --git a/cel/program.go b/cel/program.go
index 6f477afc..49bd5378 100644
--- a/cel/program.go
+++ b/cel/program.go
@@ -100,6 +100,9 @@ type EvalDetails struct {
 // State of the evaluation, non-nil if the OptTrackState or OptExhaustiveEval is specified
 // within EvalOptions.
 func (ed *EvalDetails) State() interpreter.EvalState {
+	if ed == nil {
+		return interpreter.NewEvalState()
+	}
 	return ed.state
 }
 
diff --git a/common/errors.go b/common/errors.go
index 25adc73d..89570683 100644
--- a/common/errors.go
+++ b/common/errors.go
@@ -30,9 +30,13 @@ type Errors struct {
 
 // NewErrors creates a new instance of the Errors type.
 func NewErrors(source Source) *Errors {
+	src := source
+	if src == nil {
+		src = NewTextSource("")
+	}
 	return &Errors{
 		errors:            []*Error{},
-		source:            source,
+		source:            src,
 		maxErrorsToReport: 100,
 	}
 }