diff --git a/internal/unmarshal/unmarshal.go b/internal/unmarshal/unmarshal.go
new file mode 100644
index 0000000..b54065a
--- /dev/null
+++ b/internal/unmarshal/unmarshal.go
@@ -0,0 +1,32 @@
+package unmarshal
+
+import (
+	"fmt"
+)
+
+// ErrUnexpectedType is returned when a field contains an unexpected type.
+type ErrUnexpectedType struct {
+	t interface{}
+}
+
+// Error implements the error interface.
+func (e *ErrUnexpectedType) Error() string {
+	return fmt.Sprintf("Unexpected field type: %T", e.t)
+}
+
+// GetIntStringField uses type asserstion to attempt to determine if the
+// field argument is a string or a int. Returns a string or an int or an error
+// if neither.
+func GetIntStringField(field interface{}) (string, int, error) {
+	stringField, ok := field.(string)
+	if ok {
+		return stringField, -1, nil
+	}
+
+	floatField, ok := field.(float64)
+	if !ok {
+		return "", -1, &ErrUnexpectedType{t: field}
+	}
+
+	return "", int(floatField), nil
+}
diff --git a/mailbox.go b/mailbox.go
index 33a8a3d..5db5269 100644
--- a/mailbox.go
+++ b/mailbox.go
@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/engvik/sbanken-go/internal/transport"
+	"github.com/engvik/sbanken-go/internal/unmarshal"
 )
 
 var (
@@ -55,24 +56,8 @@ func (m *MailboxMessage) UnmarshalJSON(data []byte) error {
 		return err
 	}
 
-	// getField returns either the string value or int value. Returns an error
-	// if the argument passed is neither.
-	getField := func(field interface{}) (string, int, error) {
-		stringField, ok := field.(string)
-		if ok {
-			return stringField, -1, nil
-		}
-
-		floatField, ok := field.(float64)
-		if !ok {
-			return "", -1, ErrMailboxMessageUnmarshal
-		}
-
-		return "", int(floatField), nil
-	}
-
 	// Get and set proper category value.
-	categoryString, categoryInt, err := getField(tmpMailboxMessage.Category)
+	categoryString, categoryInt, err := unmarshal.GetIntStringField(tmpMailboxMessage.Category)
 	if err != nil {
 		return err
 	}
@@ -90,7 +75,7 @@ func (m *MailboxMessage) UnmarshalJSON(data []byte) error {
 	}
 
 	// Get and set proper status value.
-	statusString, statusInt, err := getField(tmpMailboxMessage.Status)
+	statusString, statusInt, err := unmarshal.GetIntStringField(tmpMailboxMessage.Status)
 	if err != nil {
 		return err
 	}
diff --git a/transactions.go b/transactions.go
index 926d7fd..95076ef 100644
--- a/transactions.go
+++ b/transactions.go
@@ -10,6 +10,7 @@ import (
 	"time"
 
 	"github.com/engvik/sbanken-go/internal/transport"
+	"github.com/engvik/sbanken-go/internal/unmarshal"
 )
 
 var (
@@ -68,24 +69,8 @@ func (t *Transaction) UnmarshalJSON(data []byte) error {
 		return err
 	}
 
-	// getField returns either the string value or int value. Returns an error
-	// if the argument passed is neither.
-	getField := func(field interface{}) (string, int, error) {
-		stringField, ok := field.(string)
-		if ok {
-			return stringField, -1, nil
-		}
-
-		floatField, ok := field.(float64)
-		if !ok {
-			return "", -1, ErrTransactionUnmarshal
-		}
-
-		return "", int(floatField), nil
-	}
-
 	// Get and set proper source value.
-	sourceString, sourceInt, err := getField(tmpTransaction.Source)
+	sourceString, sourceInt, err := unmarshal.GetIntStringField(tmpTransaction.Source)
 	if err != nil {
 		return err
 	}