diff --git a/laymanshex.go b/laymanshex.go
index 3972878..a7f1a9b 100644
--- a/laymanshex.go
+++ b/laymanshex.go
@@ -175,6 +175,9 @@ func (part *filePart) setHandling() {
 		writeTo := func(w io.Writer, bo binary.ByteOrder, s string) {
 			buf, err := hex.DecodeString(s)
 			optPanic("invalid string of hex values: "+s, err)
+			if int64(len(buf)) > bytes {
+				panic(fmt.Sprintf("string %s is too long for byte array %s : want %d bytes", s, part.name, bytes))
+			}
 			binaryWrite(w, bo, buf)
 		}
 		part.handling = handling{bytes, readFrom, writeTo}
@@ -305,6 +308,18 @@ func backup(binpath string) {
 	fmt.Println("Created backup " + backuppath + "\n")
 }
 
+func seekErr(i int, name string, inner error) string {
+	if inner != nil {
+		namepart := ""
+		if name != "" {
+			namepart = " name: " + name
+		}
+		return fmt.Sprintf("could not advance in file (stuck at part %d%s)", i, namepart)
+	} else {
+		return ""
+	}
+}
+
 func setValues(descr fileDescription, binpath string, vals map[string]string) {
 	backup(binpath)
 	f, err := os.OpenFile(binpath, os.O_RDWR, 0666)
@@ -320,7 +335,7 @@ func setValues(descr fileDescription, binpath string, vals map[string]string) {
 		}
 		if needSeek {
 			_, err = f.Seek(part.handling.bytes, 1)
-			optPanic("set: could not advance in file (stuck at part "+strconv.Itoa(i)+"name: "+part.name+")", err)
+			optPanic(seekErr(i, part.name, err), err)
 		}
 	}
 }
@@ -333,7 +348,7 @@ func getValues(descr fileDescription, binpath string) map[string]string {
 	for i, part := range descr.parts {
 		if part.name == "" {
 			_, err = f.Seek(part.handling.bytes, 1)
-			optPanic("get: could not advance in file (stuck at part "+strconv.Itoa(i)+"name: "+part.name+")", err)
+			optPanic(seekErr(i, part.name, err), err)
 		} else {
 			vals[part.name] = part.handling.readFrom(f, descr.byteOrder)
 		}
@@ -341,7 +356,7 @@ func getValues(descr fileDescription, binpath string) map[string]string {
 	return vals
 }
 
-func padding(descr fileDescription) string {
+func fmtPadding(descr fileDescription) string {
 	pad := 0
 	for _, part := range descr.parts {
 		if n := len(part.name); n > pad {
@@ -354,7 +369,7 @@ func padding(descr fileDescription) string {
 func printValues(descr fileDescription, vals map[string]string) {
 	for _, part := range descr.parts {
 		if part.name != "" {
-			fmt.Printf("%"+padding(descr)+"s = %s\n", part.name, vals[part.name])
+			fmt.Printf("%"+fmtPadding(descr)+"s = %s\n", part.name, vals[part.name])
 		}
 	}
 }