diff --git a/bgpwtf/invoice/BUILD.bazel b/bgpwtf/invoice/BUILD.bazel
index c85bb4d..900f0b3 100644
--- a/bgpwtf/invoice/BUILD.bazel
+++ b/bgpwtf/invoice/BUILD.bazel
@@ -8,6 +8,7 @@
         "model.go",
         "render.go",
         "statusz.go",
+        "validation.go",
     ],
     importpath = "code.hackerspace.pl/hscloud/bgpwtf/invoice",
     visibility = ["//visibility:private"],
diff --git a/bgpwtf/invoice/main.go b/bgpwtf/invoice/main.go
index d93034c..5133010 100644
--- a/bgpwtf/invoice/main.go
+++ b/bgpwtf/invoice/main.go
@@ -23,42 +23,8 @@
 }
 
 func (s *service) CreateInvoice(ctx context.Context, req *pb.CreateInvoiceRequest) (*pb.CreateInvoiceResponse, error) {
-	if req.InvoiceData == nil {
-		return nil, status.Error(codes.InvalidArgument, "invoice data must be given")
-	}
-	if len(req.InvoiceData.Item) < 1 {
-		return nil, status.Error(codes.InvalidArgument, "invoice data must contain at least one item")
-	}
-	for i, item := range req.InvoiceData.Item {
-		if item.Title == "" {
-			return nil, status.Errorf(codes.InvalidArgument, "invoice data item %d must have title set", i)
-		}
-		if item.Count == 0 || item.Count > 1000000 {
-			return nil, status.Errorf(codes.InvalidArgument, "invoice data item %d must have correct count", i)
-		}
-		if item.UnitPrice == 0 {
-			return nil, status.Errorf(codes.InvalidArgument, "invoice data item %d must have correct unit price", i)
-		}
-		if item.Vat > 100000 {
-			return nil, status.Errorf(codes.InvalidArgument, "invoice data item %d must have correct vat set", i)
-		}
-	}
-	if len(req.InvoiceData.CustomerBilling) < 1 {
-		return nil, status.Error(codes.InvalidArgument, "invoice data must contain at least one line of the customer's billing address")
-	}
-	if len(req.InvoiceData.InvoicerBilling) < 1 {
-		return nil, status.Error(codes.InvalidArgument, "invoice data must contain at least one line of the invoicer's billing address")
-	}
-	for i, c := range req.InvoiceData.InvoicerContact {
-		if c.Medium == "" {
-			return nil, status.Errorf(codes.InvalidArgument, "contact point %d must have medium set", i)
-		}
-		if c.Contact == "" {
-			return nil, status.Errorf(codes.InvalidArgument, "contact point %d must have contact set", i)
-		}
-	}
-	if req.InvoiceData.InvoicerVatId == "" {
-		return nil, status.Error(codes.InvalidArgument, "invoice data must contain invoicer's vat id")
+	if err := validateInvoiceData(req.InvoiceData); err != nil {
+		return nil, status.Errorf(codes.InvalidArgument, "invoice data: %v", err)
 	}
 
 	uid, err := s.m.createInvoice(ctx, req.InvoiceData)
diff --git a/bgpwtf/invoice/validation.go b/bgpwtf/invoice/validation.go
new file mode 100644
index 0000000..d9fd820
--- /dev/null
+++ b/bgpwtf/invoice/validation.go
@@ -0,0 +1,123 @@
+package main
+
+import (
+	"fmt"
+
+	pb "code.hackerspace.pl/hscloud/bgpwtf/invoice/proto"
+)
+
+// validateGTUCodde returns a non-nil error if the given GTU (Grupy Towarów i
+// Usług) code is invalid.
+func validateGTUCode(c pb.GTUCode) error {
+	switch c {
+	case pb.GTUCode_GTU_01:
+	case pb.GTUCode_GTU_02:
+	case pb.GTUCode_GTU_03:
+	case pb.GTUCode_GTU_04:
+	case pb.GTUCode_GTU_05:
+	case pb.GTUCode_GTU_06:
+	case pb.GTUCode_GTU_07:
+	case pb.GTUCode_GTU_09:
+	case pb.GTUCode_GTU_10:
+	case pb.GTUCode_GTU_11:
+	case pb.GTUCode_GTU_12:
+	case pb.GTUCode_GTU_13:
+	default:
+		return fmt.Errorf("must be 1-13, is %d", c)
+	}
+	return nil
+}
+
+// validateGTUCodde returns a non-nil error if the given SP (Symbol Procedury)
+// code is invalid.
+func validateSPCode(c pb.SPCode) error {
+	switch c {
+	case pb.SPCode_SP_SW:
+	case pb.SPCode_SP_EE:
+	case pb.SPCode_SP_TP:
+	case pb.SPCode_SP_TT_WNT:
+	case pb.SPCode_SP_TT_D:
+	case pb.SPCode_SP_MR_T:
+	case pb.SPCode_SP_MR_UZ:
+	case pb.SPCode_SP_I_42:
+	case pb.SPCode_SP_I_63:
+	case pb.SPCode_SP_B_SPV:
+	case pb.SPCode_SP_B_SPV_DOSTAWA:
+	case pb.SPCode_SP_B_MPV_PROWIZJA:
+	case pb.SPCode_SP_MPP:
+	default:
+		return fmt.Errorf("unsupported value")
+	}
+	return nil
+}
+
+// validateItem returns a non-nil error if the given Item is invalid as part of
+// an InvoiceData when an invoice is being created.
+func validateItem(i *pb.Item) error {
+	if i.Title == "" {
+		return fmt.Errorf("must have title set")
+	}
+	if i.Count == 0 || i.Count > 1000000 {
+		return fmt.Errorf("must have correct count")
+	}
+	if i.UnitPrice == 0 {
+		return fmt.Errorf("must have correct unit price")
+	}
+	if i.Vat > 100000 {
+		return fmt.Errorf("must have correct vat set")
+	}
+	for i, code := range i.GtuCode {
+		if err := validateGTUCode(code); err != nil {
+			return fmt.Errorf("GTU code %d: %v", i, err)
+		}
+	}
+	return nil
+}
+
+// validateContactPoint returns a non-nil error if the given ContactPoint is
+// invalid as part of an InvoiceData when an invoice is being created.
+func validateContactPoint(cp *pb.ContactPoint) error {
+	if cp.Medium == "" {
+		return fmt.Errorf("must have medium set")
+	}
+	if cp.Contact == "" {
+		return fmt.Errorf("must have contact set")
+	}
+	return nil
+}
+
+// validateInvoiceData returns a non-nil error if the given InvoiceData cannot
+// be used to createa new invoice.
+func validateInvoiceData(id *pb.InvoiceData) error {
+	if id == nil {
+		return fmt.Errorf("must be given")
+	}
+	if len(id.Item) < 1 {
+		return fmt.Errorf("must contain at least one item")
+	}
+	for i, item := range id.Item {
+		if err := validateItem(item); err != nil {
+			return fmt.Errorf("invoice data item %d: %v", i, err)
+		}
+	}
+	if len(id.CustomerBilling) < 1 {
+		return fmt.Errorf("must contain at least one line of the customer's billing address")
+	}
+	if len(id.InvoicerBilling) < 1 {
+		return fmt.Errorf("must contain at least one line of the invoicer's billing address")
+	}
+	for i, c := range id.InvoicerContact {
+		if err := validateContactPoint(c); err != nil {
+			return fmt.Errorf("contact point %d: %v", i, err)
+		}
+	}
+	if id.InvoicerVatId == "" {
+		return fmt.Errorf("must contain invoicer's vat id")
+	}
+	for i, code := range id.SpCode {
+		if err := validateSPCode(code); err != nil {
+			return fmt.Errorf("SP code %d: %v", i, err)
+		}
+	}
+	return nil
+}