diff --git a/asserter/asserter.go b/asserter/asserter.go
index da68bfe6..3cd9029e 100644
--- a/asserter/asserter.go
+++ b/asserter/asserter.go
@@ -16,10 +16,17 @@ package asserter
 
 import (
 	"context"
+	"errors"
 
 	"github.com/coinbase/rosetta-sdk-go/types"
 )
 
+var (
+	// ErrAsserterNotInitialized is returned when some call in the asserter
+	// package requires the asserter to be initialized first.
+	ErrAsserterNotInitialized = errors.New("asserter not initialized")
+)
+
 // Asserter contains all logic to perform static
 // validation on Rosetta Server responses.
 type Asserter struct {
@@ -81,12 +88,3 @@ func NewWithOptions(
 
 	return asserter
 }
-
-func (a *Asserter) operationStatuses() []string {
-	statuses := []string{}
-	for k := range a.operationStatusMap {
-		statuses = append(statuses, k)
-	}
-
-	return statuses
-}
diff --git a/asserter/asserter_test.go b/asserter/asserter_test.go
index fdf91e97..3f9d861b 100644
--- a/asserter/asserter_test.go
+++ b/asserter/asserter_test.go
@@ -1,3 +1,17 @@
+// Copyright 2020 Coinbase, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 package asserter
 
 import (
diff --git a/asserter/block.go b/asserter/block.go
index cdd2eb03..a7a8aa60 100644
--- a/asserter/block.go
+++ b/asserter/block.go
@@ -106,7 +106,7 @@ func AccountIdentifier(account *types.AccountIdentifier) error {
 // AFTER an operation has been validated.
 func (a *Asserter) OperationSuccessful(operation *types.Operation) (bool, error) {
 	if a == nil {
-		return false, errors.New("must initialize asserter")
+		return false, ErrAsserterNotInitialized
 	}
 
 	val, ok := a.operationStatusMap[operation.Status]
@@ -117,6 +117,21 @@ func (a *Asserter) OperationSuccessful(operation *types.Operation) (bool, error)
 	return val, nil
 }
 
+// operationStatuses returns all operation statuses the
+// asserter consider valid.
+func (a *Asserter) operationStatuses() ([]string, error) {
+	if a == nil {
+		return nil, ErrAsserterNotInitialized
+	}
+
+	statuses := []string{}
+	for k := range a.operationStatusMap {
+		statuses = append(statuses, k)
+	}
+
+	return statuses, nil
+}
+
 // Operation ensures a types.Operation has a valid
 // type, status, and amount.
 func (a *Asserter) Operation(
@@ -124,7 +139,7 @@ func (a *Asserter) Operation(
 	index int64,
 ) error {
 	if a == nil {
-		return errors.New("must initialize asserter")
+		return ErrAsserterNotInitialized
 	}
 
 	if operation == nil {
@@ -139,7 +154,12 @@ func (a *Asserter) Operation(
 		return fmt.Errorf("Operation.Type %s is invalid", operation.Type)
 	}
 
-	if operation.Status == "" || !contains(a.operationStatuses(), operation.Status) {
+	validOperationStatuses, err := a.operationStatuses()
+	if err != nil {
+		return err
+	}
+
+	if operation.Status == "" || !contains(validOperationStatuses, operation.Status) {
 		return fmt.Errorf("Operation.Status %s is invalid", operation.Status)
 	}
 
@@ -213,7 +233,7 @@ func (a *Asserter) Transaction(
 	transaction *types.Transaction,
 ) error {
 	if a == nil {
-		return errors.New("must initialize asserter")
+		return ErrAsserterNotInitialized
 	}
 
 	if transaction == nil {
@@ -248,7 +268,7 @@ func (a *Asserter) Block(
 	block *types.Block,
 ) error {
 	if a == nil {
-		return errors.New("must initialize asserter")
+		return ErrAsserterNotInitialized
 	}
 
 	if block == nil {