diff --git a/packages/node/src/assignment/assignment.ts b/packages/node/src/assignment/assignment.ts
index 6bcd162..8944b63 100644
--- a/packages/node/src/assignment/assignment.ts
+++ b/packages/node/src/assignment/assignment.ts
@@ -1,6 +1,7 @@
 import { EvaluationVariant } from '@amplitude/experiment-core';
 
 import { ExperimentUser } from '../types/user';
+import { safeStringTrim } from '../util/string';
 
 /**
  * @deprecated Assignment tracking is deprecated. Use Exposure tracking.
@@ -35,11 +36,14 @@ export class Assignment {
   }
 
   public canonicalize(): string {
-    let canonical = `${this.user.user_id?.trim()} ${this.user.device_id?.trim()} `;
+    let canonical = `${safeStringTrim(this.user.user_id)} ${safeStringTrim(
+      this.user.device_id,
+    )} `;
     for (const key of Object.keys(this.results).sort()) {
       const variant = this.results[key];
       if (variant?.key) {
-        canonical += key.trim() + ' ' + variant?.key?.trim() + ' ';
+        canonical +=
+          safeStringTrim(key) + ' ' + safeStringTrim(variant.key) + ' ';
       }
     }
     return canonical;
diff --git a/packages/node/src/exposure/exposure.ts b/packages/node/src/exposure/exposure.ts
index 8293d6a..28b378d 100644
--- a/packages/node/src/exposure/exposure.ts
+++ b/packages/node/src/exposure/exposure.ts
@@ -1,6 +1,7 @@
 import { EvaluationVariant } from '@amplitude/experiment-core';
 
 import { ExperimentUser } from '../types/user';
+import { safeStringTrim } from '../util/string';
 
 export interface ExposureService {
   track(exposure: Exposure): Promise<void>;
@@ -29,11 +30,14 @@ export class Exposure {
   }
 
   public canonicalize(): string {
-    let canonical = `${this.user.user_id?.trim()} ${this.user.device_id?.trim()} `;
+    let canonical = `${safeStringTrim(this.user.user_id)} ${safeStringTrim(
+      this.user.device_id,
+    )} `;
     for (const key of Object.keys(this.results).sort()) {
       const variant = this.results[key];
       if (variant?.key) {
-        canonical += key.trim() + ' ' + variant?.key?.trim() + ' ';
+        canonical +=
+          safeStringTrim(key) + ' ' + safeStringTrim(variant.key) + ' ';
       }
     }
     return canonical;
diff --git a/packages/node/src/util/string.ts b/packages/node/src/util/string.ts
new file mode 100644
index 0000000..f7786fa
--- /dev/null
+++ b/packages/node/src/util/string.ts
@@ -0,0 +1,15 @@
+/**
+ * Safely converts a value to a trimmed string.
+ * Returns empty string for null/undefined values.
+ * @param value - The value to convert
+ * @returns Trimmed string representation
+ */
+export function safeStringTrim(value: unknown): string {
+  if (value == null) {
+    return '';
+  }
+  if (typeof value === 'string') {
+    return value.trim();
+  }
+  return String(value).trim();
+}
diff --git a/packages/node/test/assignment/assignment.test.ts b/packages/node/test/assignment/assignment.test.ts
new file mode 100644
index 0000000..9e064d5
--- /dev/null
+++ b/packages/node/test/assignment/assignment.test.ts
@@ -0,0 +1,32 @@
+import { Assignment } from 'src/assignment/assignment';
+import { ExperimentUser } from 'src/types/user';
+
+describe('Assignment.canonicalize()', () => {
+  test('canonicalizes with string user_id and device_id', () => {
+    const user: ExperimentUser = {
+      user_id: 'user123',
+      device_id: 'device456',
+    };
+    const results = {
+      'flag-key-1': { key: 'on', value: 'on' },
+      'flag-key-2': { key: 'control', value: 'control' },
+    };
+    const assignment = new Assignment(user, results);
+    expect(assignment.canonicalize()).toBe(
+      'user123 device456 flag-key-1 on flag-key-2 control ',
+    );
+  });
+
+  test('handles non-string user_id and device_id without throwing', () => {
+    const user: ExperimentUser = {
+      user_id: 12345 as any,
+      device_id: 67890 as any,
+    };
+    const results = {
+      'flag-key-1': { key: 999 as any, value: 'on' },
+    };
+    const assignment = new Assignment(user, results);
+    expect(() => assignment.canonicalize()).not.toThrow();
+    expect(assignment.canonicalize()).toBe('12345 67890 flag-key-1 999 ');
+  });
+});
diff --git a/packages/node/test/exposure/exposure.test.ts b/packages/node/test/exposure/exposure.test.ts
new file mode 100644
index 0000000..fe560a4
--- /dev/null
+++ b/packages/node/test/exposure/exposure.test.ts
@@ -0,0 +1,32 @@
+import { Exposure } from 'src/exposure/exposure';
+import { ExperimentUser } from 'src/types/user';
+
+describe('Exposure.canonicalize()', () => {
+  test('canonicalizes with string user_id and device_id', () => {
+    const user: ExperimentUser = {
+      user_id: 'user123',
+      device_id: 'device456',
+    };
+    const results = {
+      'flag-key-1': { key: 'on', value: 'on' },
+      'flag-key-2': { key: 'control', value: 'control' },
+    };
+    const exposure = new Exposure(user, results);
+    expect(exposure.canonicalize()).toBe(
+      'user123 device456 flag-key-1 on flag-key-2 control ',
+    );
+  });
+
+  test('handles non-string user_id and device_id without throwing', () => {
+    const user: ExperimentUser = {
+      user_id: 12345 as any,
+      device_id: 67890 as any,
+    };
+    const results = {
+      'flag-key-1': { key: 999 as any, value: 'on' },
+    };
+    const exposure = new Exposure(user, results);
+    expect(() => exposure.canonicalize()).not.toThrow();
+    expect(exposure.canonicalize()).toBe('12345 67890 flag-key-1 999 ');
+  });
+});
diff --git a/packages/node/test/util/string.test.ts b/packages/node/test/util/string.test.ts
new file mode 100644
index 0000000..29c85b1
--- /dev/null
+++ b/packages/node/test/util/string.test.ts
@@ -0,0 +1,15 @@
+import { safeStringTrim } from 'src/util/string';
+
+describe('safeStringTrim', () => {
+  test('trims string values', () => {
+    expect(safeStringTrim('  hello  ')).toBe('hello');
+    expect(safeStringTrim('test')).toBe('test');
+  });
+
+  test('converts non-string values to trimmed strings', () => {
+    expect(safeStringTrim(123)).toBe('123');
+    expect(safeStringTrim(null)).toBe('');
+    expect(safeStringTrim(undefined)).toBe('');
+    expect(safeStringTrim(true)).toBe('true');
+  });
+});