diff --git a/internal/pattern/pattern.go b/internal/pattern/pattern.go
index 68cfb81..405f57b 100644
--- a/internal/pattern/pattern.go
+++ b/internal/pattern/pattern.go
@@ -30,9 +30,10 @@ func FormatStrings() []string {
 }
 
 var (
-	ErrInvalidHeader    = errors.New("invalid header")
-	ErrUnknownExtension = errors.New("unknown pattern extension")
-	ErrPatternTooBig    = errors.New("pattern too big")
+	ErrInvalidHeader       = errors.New("invalid header")
+	ErrUnknownExtension    = errors.New("unknown pattern extension")
+	ErrPatternTooBig       = errors.New("pattern too big")
+	ErrUnexpectedCharacter = errors.New("unexpected character")
 )
 
 const (
diff --git a/internal/pattern/plaintext.go b/internal/pattern/plaintext.go
index 9766649..b639a40 100644
--- a/internal/pattern/plaintext.go
+++ b/internal/pattern/plaintext.go
@@ -40,6 +40,8 @@ func UnmarshalPlaintext(r io.Reader) (Pattern, error) {
 				case 'O', '*':
 					tileLine[x] = 1
 					x++
+				default:
+					return pattern, fmt.Errorf("plaintext: %w: %q in line %q", ErrUnexpectedCharacter, string(b), line)
 				}
 			}
 			if len(tileLine) > largest {