diff --git a/cmd/proofgen/main.go b/cmd/proofgen/main.go
index 9a5bb228..ba56f62e 100644
--- a/cmd/proofgen/main.go
+++ b/cmd/proofgen/main.go
@@ -402,9 +402,9 @@ func staticConsistencyProbes(dir string) error {
 	tests := []consistencyProbe{
 		{0, 0, root1, root2, proof1, "sizes are equal (zero) but roots are not", true},
 		{1, 1, root1, root2, proof1, "sizes are equal (one) but roots are not", true},
-		// Sizes that are always consistent.
-		{0, 0, root1, root1, proof1, "sizes are equal (zero) and proof is empty", false},
+		{0, 0, root1, root1, proof1, "sizes are equal (zero) and proof is empty", true},
 		{0, 1, root1, root2, proof1, "size1 is zero and does not equal size2", true},
+		// Sizes that are always consistent.
 		{1, 1, root2, root2, proof1, "sizes are equal (one) and proof is empty", false},
 		// Time travel to the past.
 		{1, 0, root1, root2, proof1, "size1 is greater than size2", true},
@@ -418,6 +418,7 @@ func staticConsistencyProbes(dir string) error {
 		{0, 0, sha256EmptyTreeHash, sha256EmptyTreeHash, proof2, "sizes match but proof is not empty and sizes are zero", true},
 		{1, 1, sha256EmptyTreeHash, sha256EmptyTreeHash, proof2, "sizes match but proof is not empty and sizes are one", true},
 		// Fail to validate empty tree
+		{0, 1, sha256EmptyTreeHash, root2, proof1, "size1 is zero and size2 is not zero", true},
 		{0, 1, sha256EmptyTreeHash, sha256EmptyTreeHash, proof2, "consistency check on empty tree (size1 is zero) is useless", true},
 	}
 
diff --git a/proof/proof.go b/proof/proof.go
index df528770..24c221bd 100644
--- a/proof/proof.go
+++ b/proof/proof.go
@@ -82,12 +82,15 @@ func SubtreeInclusion(index, start, end uint64) (Nodes, error) {
 
 // Consistency returns the information on how to fetch and construct a
 // consistency proof between the two given tree sizes of a log Merkle tree. It
-// requires 0 <= size1 <= size2.
+// requires 0 < size1 <= size2.
 func Consistency(size1, size2 uint64) (Nodes, error) {
 	if size1 > size2 {
 		return Nodes{}, fmt.Errorf("tree size %d > %d", size1, size2)
 	}
-	if size1 == size2 || size1 == 0 {
+	if size1 == 0 {
+		return Nodes{}, fmt.Errorf("consistency proof from empty tree is meaningless")
+	}
+	if size1 == size2 {
 		return Nodes{IDs: []compact.NodeID{}}, nil
 	}
 
diff --git a/proof/proof_test.go b/proof/proof_test.go
index 2984a702..41393fc2 100644
--- a/proof/proof_test.go
+++ b/proof/proof_test.go
@@ -341,6 +341,8 @@ func TestConsistency(t *testing.T) {
 		// Errors.
 		{size1: 5, size2: 0, wantErr: true},
 		{size1: 9, size2: 8, wantErr: true},
+		{size1: 0, size2: 5, wantErr: true},
+		{size1: 0, size2: 0, wantErr: true},
 
 		{size1: 1, size2: 2, want: nodes(id(0, 1))},                            // b
 		{size1: 1, size2: 4, want: nodes(id(0, 1), id(1, 1))},                  // b h
diff --git a/proof/verify.go b/proof/verify.go
index 1c9186d4..24d9e049 100644
--- a/proof/verify.go
+++ b/proof/verify.go
@@ -112,13 +112,13 @@ func RootFromConsistencyProof(hasher merkle.LogHasher, size1, size2 uint64, proo
 	switch {
 	case size2 < size1:
 		return nil, fmt.Errorf("size2 (%d) < size1 (%d)", size1, size2)
+	case size1 == 0:
+		return nil, errors.New("consistency proof from empty tree is meaningless")
 	case size1 == size2:
 		if len(proof) > 0 {
 			return nil, errors.New("size1=size2, but proof is not empty")
 		}
 		return root1, nil
-	case size1 == 0:
-		return nil, errors.New("consistency proof from empty tree is meaningless")
 	case len(proof) == 0:
 		return nil, errors.New("empty proof")
 	}
diff --git a/testdata/consistency/additional/size1-is-zero-and-size2-is-not-zero.json b/testdata/consistency/additional/size1-is-zero-and-size2-is-not-zero.json
new file mode 100644
index 00000000..a054a889
--- /dev/null
+++ b/testdata/consistency/additional/size1-is-zero-and-size2-is-not-zero.json
@@ -0,0 +1,9 @@
+{
+  "size1": 0,
+  "size2": 1,
+  "root1": "47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=",
+  "root2": "ZG9uJ3QgY2FyZSAy",
+  "proof": [],
+  "desc": "size1 is zero and size2 is not zero",
+  "wantErr": true
+}
\ No newline at end of file
diff --git a/testdata/consistency/additional/sizes-are-equal-zero-and-proof-is-empty.json b/testdata/consistency/additional/sizes-are-equal-zero-and-proof-is-empty.json
index 7c4ac437..6e5706e3 100644
--- a/testdata/consistency/additional/sizes-are-equal-zero-and-proof-is-empty.json
+++ b/testdata/consistency/additional/sizes-are-equal-zero-and-proof-is-empty.json
@@ -5,5 +5,5 @@
   "root2": "ZG9uJ3QgY2FyZSAx",
   "proof": [],
   "desc": "sizes are equal (zero) and proof is empty",
-  "wantErr": false
+  "wantErr": true
 }
\ No newline at end of file
diff --git a/testonly/tree_fuzz_test.go b/testonly/tree_fuzz_test.go
index 392dfdd3..fe0a9968 100644
--- a/testonly/tree_fuzz_test.go
+++ b/testonly/tree_fuzz_test.go
@@ -28,7 +28,7 @@ func FuzzConsistencyProofAndVerify(f *testing.F) {
 		if begin > end || end > size {
 			return
 		}
-		if begin == 0 && end > 0 {
+		if begin == 0 {
 			return
 		}
 		tree := newTree(genEntries(size))
@@ -176,6 +176,9 @@ func FuzzConsistencyProofAgainstReferenceImplementation(f *testing.F) {
 		if begin > end || end > size {
 			return
 		}
+		if begin == 0 {
+			return
+		}
 		entries := genEntries(size)
 		tree := newTree(entries)
 		got, err := tree.ConsistencyProof(begin, end)
diff --git a/testonly/tree_test.go b/testonly/tree_test.go
index a6120545..8eb1108b 100644
--- a/testonly/tree_test.go
+++ b/testonly/tree_test.go
@@ -154,8 +154,11 @@ func TestTreeConsistencyProof(t *testing.T) {
 	if _, err := mt.ConsistencyProof(6, 3); err == nil {
 		t.Error("ConsistencyProof(6, 3) succeeded unexpectedly")
 	}
+	if _, err := mt.ConsistencyProof(0, 3); err == nil {
+		t.Error("ConsistencyProof(0, 3) succeeded unexpectedly")
+	}
 
-	for size1 := range uint64(8) + 1 {
+	for size1 := uint64(1); size1 <= 8; size1++ {
 		for size2 := size1; size2 <= 8; size2++ {
 			t.Run(fmt.Sprintf("%d:%d", size1, size2), func(t *testing.T) {
 				got, err := mt.ConsistencyProof(size1, size2)
@@ -178,8 +181,8 @@ func TestTreeConsistencyProofFuzz(t *testing.T) {
 	for treeSize := uint64(1); treeSize <= 256; treeSize++ {
 		mt := newTree(entries[:treeSize])
 		for range 8 {
-			size2 := rand.Uint64N(treeSize + 1)
-			size1 := rand.Uint64N(size2 + 1)
+			size2 := rand.Uint64N(treeSize) + 1
+			size1 := rand.Uint64N(size2) + 1
 
 			got, err := mt.ConsistencyProof(size1, size2)
 			if err != nil {