diff --git a/client.go b/client.go
index 4988e3c..b049d2e 100644
--- a/client.go
+++ b/client.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"strings"
 	"sync/atomic"
 	"time"
 
@@ -54,6 +55,10 @@ func NewClient(apiKey string, options ...Option) *Client {
 	c.client.SetLogger(c.log)
 
 	if c.config.localEvaluation {
+		if !strings.HasPrefix(apiKey, "ser.") {
+			panic("In order to use local evaluation, please generate a server key in the environment settings page.")
+		}
+
 		go c.pollEnvironment(c.ctxLocalEval)
 	}
 	// Initialize analytics processor
diff --git a/client_test.go b/client_test.go
index 4607e53..68f5273 100644
--- a/client_test.go
+++ b/client_test.go
@@ -16,6 +16,13 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+func TestClientErrorsIfLocalEvaluationWithNonServerSideKey(t *testing.T) {
+	// When, Then
+	assert.Panics(t, func() {
+		_ = flagsmith.NewClient("key", flagsmith.WithLocalEvaluation(context.Background()))
+	})
+}
+
 func TestClientUpdatesEnvironmentOnStartForLocalEvaluation(t *testing.T) {
 	// Given
 	ctx := context.Background()
diff --git a/fixtures/fixture.go b/fixtures/fixture.go
index 6876f9d..805fa0f 100644
--- a/fixtures/fixture.go
+++ b/fixtures/fixture.go
@@ -6,7 +6,7 @@ import (
 )
 
 const BaseURL = "http://localhost:8000/api/v1/"
-const EnvironmentAPIKey = "test_key"
+const EnvironmentAPIKey = "ser.test_key"
 const Feature1Value = "some_value"
 const Feature1Name = "feature_1"
 const Feature1ID = 1