diff --git a/error.go b/error.go
index 80e039a..d84c1a3 100644
--- a/error.go
+++ b/error.go
@@ -2,10 +2,11 @@ package step
 
 // Error enumerates errors common to this package.
 type Error string; const (
-	ErrCircularInheritance     Error = "Circular inheritance"
-	ErrFrontMatterMalformed    Error = "Front matter is malformed"
-	ErrFrontMatterNeverClosed  Error = "Front matter is never closed"
-	ErrFrontMatterDuplicateKey Error = "Duplicate key in front matter"
+	ErrCircularInheritance     Error = "circular inheritance"
+	ErrFrontMatterMalformed    Error = "front matter is malformed"
+	ErrFrontMatterNeverClosed  Error = "front matter is never closed"
+	ErrFrontMatterDuplicateKey Error = "duplicate key in front matter"
+	ErrTypeMismatch            Error = "type mismatch"
 )
 
 // Error fulfills the error interface.