diff --git a/cryptobyte/builder.go b/cryptobyte/builder.go
index d0c6c3f..ca7b1db 100644
--- a/cryptobyte/builder.go
+++ b/cryptobyte/builder.go
@@ -274,9 +274,11 @@ func (b *Builder) flushChild() {
 		return
 	}
 
-	if !b.fixedSize {
-		b.result = child.result // In case child reallocated result.
+	if b.fixedSize && &b.result[0] != &child.result[0] {
+		panic("cryptobyte: BuilderContinuation reallocated a fixed-size buffer")
 	}
+
+	b.result = child.result
 }
 
 func (b *Builder) add(bytes ...byte) {
@@ -284,7 +286,7 @@ func (b *Builder) add(bytes ...byte) {
 		return
 	}
 	if b.child != nil {
-		panic("attempted write while child is pending")
+		panic("cryptobyte: attempted write while child is pending")
 	}
 	if len(b.result)+len(bytes) < len(bytes) {
 		b.err = errors.New("cryptobyte: length overflow")
@@ -304,7 +306,7 @@ func (b *Builder) Unwrite(n int) {
 		return
 	}
 	if b.child != nil {
-		panic("attempted unwrite while child is pending")
+		panic("cryptobyte: attempted unwrite while child is pending")
 	}
 	length := len(b.result) - b.pendingLenLen - b.offset
 	if length < 0 {
diff --git a/cryptobyte/cryptobyte_test.go b/cryptobyte/cryptobyte_test.go
index b859cc9..fe658ef 100644
--- a/cryptobyte/cryptobyte_test.go
+++ b/cryptobyte/cryptobyte_test.go
@@ -412,6 +412,33 @@ func TestUnwrite(t *testing.T) {
 	})
 }
 
+func TestFixedBuilderLengthPrefixed(t *testing.T) {
+	bufCap := 10
+	inner := bytes.Repeat([]byte{0xff}, bufCap-2)
+	buf := make([]byte, 0, bufCap)
+	b := NewFixedBuilder(buf)
+	b.AddUint16LengthPrefixed(func(b *Builder) {
+		b.AddBytes(inner)
+	})
+	if got := b.BytesOrPanic(); len(got) != bufCap {
+		t.Errorf("Expected output lenght to be %d, got %d", bufCap, len(got))
+	}
+}
+
+func TestFixedBuilderPanicReallocate(t *testing.T) {
+	defer func() {
+		recover()
+	}()
+
+	b := NewFixedBuilder(make([]byte, 0, 10))
+	b1 := NewFixedBuilder(make([]byte, 0, 10))
+	b.AddUint16LengthPrefixed(func(b *Builder) {
+		*b = *b1
+	})
+
+	t.Error("Builder did not panic")
+}
+
 // ASN.1
 
 func TestASN1Int64(t *testing.T) {