diff --git a/mock/requestbuilder.go b/mock/requestbuilder.go
index 3a7836b..3e675a0 100644
--- a/mock/requestbuilder.go
+++ b/mock/requestbuilder.go
@@ -2,6 +2,7 @@ package mock
 
 import (
 	"encoding/base64"
+	"net/http"
 )
 
 type RequestBuilder struct {
@@ -57,9 +58,14 @@ func createBasicToken(username string, password string) string {
 	return basicToken
 }
 
+// AddHeader Sets a request header key and values to match against on the mock definition
+//
+// Note: the header key will be formatted to [http.CanonicalHeaderKey] as smocker is case-sensitive regarding header
+// keys, this will help prevent mock expectations not matching as Go's standard http library uses the Canonical format.
 func (rb RequestBuilder) AddHeader(key string, values ...string) RequestBuilder {
 	rb.initialiseHeaders()
-	rb.request.Headers[key] = values
+	canonicalHeaderKey := http.CanonicalHeaderKey(key)
+	rb.request.Headers[canonicalHeaderKey] = values
 
 	return rb
 }
diff --git a/mock/requestbuilder_test.go b/mock/requestbuilder_test.go
index e95ced7..3b25e54 100644
--- a/mock/requestbuilder_test.go
+++ b/mock/requestbuilder_test.go
@@ -75,8 +75,9 @@ func TestNewRequestBuilder_AddQueryParam_Build(t *testing.T) {
 
 func TestNewRequestBuilder_AddHeader_Build(t *testing.T) {
 	expectedHeaders := map[string][]string{
-		"Content-Type":  {"application/json", "application/vnd.api+json"},
-		"Authorization": {"Bearer sv2361fr1o8ph3oin"},
+		"Content-Type":            {"application/json", "application/vnd.api+json"},
+		"Authorization":           {"Bearer sv2361fr1o8ph3oin"},
+		"Canonical-Header-Format": {"some-value"},
 	}
 	expectedRequest := mock.Request{
 		Method:  http.MethodPut,
@@ -87,6 +88,7 @@ func TestNewRequestBuilder_AddHeader_Build(t *testing.T) {
 	requestBuilder := mock.NewRequestBuilder(http.MethodPut, "/foo/bar")
 	requestBuilder.AddHeader("Content-Type", "application/json", "application/vnd.api+json")
 	requestBuilder.AddHeader("Authorization", "Bearer sv2361fr1o8ph3oin")
+	requestBuilder.AddHeader("canonical-header-FORMAT", "some-value")
 	request := requestBuilder.Build()
 
 	assert.Equal(t, expectedRequest, request)