diff --git a/internal/enclave/pcr.go b/internal/enclave/pcr.go
index 05e0029..1bc60ce 100644
--- a/internal/enclave/pcr.go
+++ b/internal/enclave/pcr.go
@@ -34,18 +34,17 @@ func (p PCR) String() string {
 }
 
 // Equal returns true if (and only if) the two given PCR maps are identical.
+// Note that this function ignores PCR4 because it contains a hash over the
+// parent's instance ID, which is only known at runtime. We ignore it for now.
 func (ours PCR) Equal(theirs PCR) bool {
-	// PCR4 contains a hash over the parent's instance ID, which is known at
-	// runtime.  We ignore it for now, until we have a better solution for how
-	// to handle this.
-	delete(ours, 4)
-	delete(theirs, 4)
-
-	if len(ours) != len(theirs) {
+	if pcrLen(ours) != pcrLen(theirs) {
 		return false
 	}
 
 	for i, ourValue := range ours {
+		if i == 4 {
+			continue
+		}
 		theirValue, exists := theirs[i]
 		if !exists {
 			return false
@@ -56,3 +55,11 @@ func (ours PCR) Equal(theirs PCR) bool {
 	}
 	return true
 }
+
+func pcrLen(p PCR) int {
+	n := len(p)
+	if _, ok := p[4]; ok {
+		n--
+	}
+	return n
+}
diff --git a/internal/enclave/pcr_test.go b/internal/enclave/pcr_test.go
index 81e444e..322c885 100644
--- a/internal/enclave/pcr_test.go
+++ b/internal/enclave/pcr_test.go
@@ -91,26 +91,26 @@ func TestPCRsEqual(t *testing.T) {
 			want: true,
 		},
 		{
-			name: "length mismatch",
+			name: "ignore PCR4 if only set once",
 			pcr1: PCR{
-				1: []byte("foobar"),
-				2: []byte("foo"),
+				1: []byte("foo"),
+				4: []byte("bar"),
 			},
 			pcr2: PCR{
-				1: []byte("foobar"),
+				1: []byte("foo"),
 			},
-			want: false,
+			want: true,
 		},
 		{
-			name: "length mismatch due to PCR4",
+			name: "length mismatch",
 			pcr1: PCR{
-				1: []byte("foo"),
-				4: []byte("bar"),
+				1: []byte("foobar"),
+				2: []byte("foo"),
 			},
 			pcr2: PCR{
-				1: []byte("foo"),
+				1: []byte("foobar"),
 			},
-			want: true,
+			want: false,
 		},
 		{
 			name: "PCR index mismatch",