diff --git a/cmd/veil/main_test.go b/cmd/veil/main_test.go
index ea32314..f9aecf0 100644
--- a/cmd/veil/main_test.go
+++ b/cmd/veil/main_test.go
@@ -268,9 +268,12 @@ func TestAttestation(t *testing.T) {
 			var a enclave.AttestationDoc
 			require.NoError(t, json.Unmarshal(body, &a))
 
-			// "Verify" the attestation document using our noop attester.
+			// Verify the attestation document.  We expect no error but if the
+			// test is run inside a Nitro Enclave, we will get ErrDebugMode.
 			aux, err := attester.Verify(&a, c.nonce)
-			require.NoError(t, err, errFromBody(t, resp))
+			if err != nil {
+				require.ErrorIs(t, err, nitro.ErrDebugMode, errFromBody(t, resp))
+			}
 
 			// Ensure that the recovered nonce matches what we sent.
 			n, err := attestation.GetNonce(aux)
diff --git a/internal/enclave/nitro/attester.go b/internal/enclave/nitro/attester.go
index e56c31c..bc096eb 100644
--- a/internal/enclave/nitro/attester.go
+++ b/internal/enclave/nitro/attester.go
@@ -13,6 +13,7 @@ import (
 )
 
 var _ enclave.Attester = (*Attester)(nil)
+var ErrDebugMode = errors.New("attestation document was produced in debug mode")
 
 // Attester implements the attester interface by drawing on the AWS Nitro
 // Enclave hypervisor.
@@ -102,8 +103,6 @@ func (a *Attester) Verify(
 		return nil, err
 	}
 
-	// TODO: return an error if the attestation mode was produced in debug mode.
-
 	// Verify that the attestation document contains the nonce that we may have
 	// asked it to embed.
 	if ourNonce != nil {
@@ -116,9 +115,15 @@ func (a *Attester) Verify(
 		}
 	}
 
+	// If the enclave is running in debug mode, return an error *and* the
+	// auxiliary information.
+	if res.Document.PCRs.FromDebugMode() {
+		err = ErrDebugMode
+	}
+
 	return &enclave.AuxInfo{
 		Nonce:     convertFrom(res.Document.Nonce),
 		UserData:  convertFrom(res.Document.UserData),
 		PublicKey: convertFrom(res.Document.PublicKey),
-	}, nil
+	}, err
 }
diff --git a/internal/enclave/nitro/pcr.go b/internal/enclave/nitro/pcr.go
index 8b0ffc4..fc14ea6 100644
--- a/internal/enclave/nitro/pcr.go
+++ b/internal/enclave/nitro/pcr.go
@@ -2,11 +2,14 @@ package nitro
 
 import (
 	"bytes"
+	"crypto/sha512"
 
 	"github.com/Amnesic-Systems/veil/internal/enclave"
 	"github.com/Amnesic-Systems/veil/internal/errs"
 )
 
+var emptyPCR = make([]byte, sha512.Size384)
+
 // pcr represents the enclave's platform configuration register (PCR) values.
 type pcr map[uint][]byte
 
@@ -27,6 +30,16 @@ func getPCRs() (_ pcr, err error) {
 	return pcr(res.Document.PCRs), nil
 }
 
+// FromDebugMode returns true if the given PCR map was generated by an enclave
+// in debug mode.
+func (p pcr) FromDebugMode() bool {
+	// PCRs 0, 1, and 2 are set at compile time but will be unset if the enclave
+	// is running in debug mode.
+	return bytes.Equal(p[0], emptyPCR) &&
+		bytes.Equal(p[1], emptyPCR) &&
+		bytes.Equal(p[2], emptyPCR)
+}
+
 // Equal returns true if (and only if) the two given PCR maps are identical.
 func (ours pcr) Equal(theirs pcr) bool {
 	// PCR4 contains a hash over the parent's instance ID.  Our enclaves run
diff --git a/internal/enclave/nitro/pcr_test.go b/internal/enclave/nitro/pcr_test.go
index 30da23c..c9161e8 100644
--- a/internal/enclave/nitro/pcr_test.go
+++ b/internal/enclave/nitro/pcr_test.go
@@ -27,6 +27,44 @@ func TestGetPCRs(t *testing.T) {
 	require.Equal(t, pcrs1, pcrs2)
 }
 
+func TestPCRsFromDebugMode(t *testing.T) {
+	cases := []struct {
+		name string
+		pcrs pcr
+		want bool
+	}{
+		{
+			name: "empty",
+			pcrs: pcr{},
+		},
+		{
+			name: "debug mode",
+			pcrs: pcr{
+				0: emptyPCR,
+				1: emptyPCR,
+				2: emptyPCR,
+				3: []byte("foo"), // Should be ignored.
+				4: []byte("bar"), // Should be ignored.
+			},
+			want: true,
+		},
+		{
+			name: "not debug mode",
+			pcrs: pcr{
+				0: []byte("foo"),
+				1: emptyPCR,
+				2: emptyPCR,
+			},
+		},
+	}
+
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			require.Equal(t, c.want, c.pcrs.FromDebugMode())
+		})
+	}
+}
+
 func TestPCRsEqual(t *testing.T) {
 	cases := []struct {
 		name string
diff --git a/internal/service/attestation/aux_test.go b/internal/service/attestation/aux_test.go
index 6d13acb..494db37 100644
--- a/internal/service/attestation/aux_test.go
+++ b/internal/service/attestation/aux_test.go
@@ -130,8 +130,12 @@ func TestBuilder(t *testing.T) {
 			doc, err := b.Attest(c.attestFields...)
 			require.NoError(t, err)
 
+			// Verify the attestation document.  We expect no error but if the
+			// test is run inside a Nitro Enclave, we will get ErrDebugMode.
 			aux, err := attester.Verify(doc, nil)
-			require.NoError(t, err)
+			if err != nil {
+				require.ErrorIs(t, err, nitro.ErrDebugMode)
+			}
 			require.Equal(t, c.wantAux, aux)
 		})
 	}