diff --git a/cas/src/main/java/org/springframework/security/cas/authentication/CasAuthenticationToken.java b/cas/src/main/java/org/springframework/security/cas/authentication/CasAuthenticationToken.java
index dfd1c4eaa3a..7557728b07d 100644
--- a/cas/src/main/java/org/springframework/security/cas/authentication/CasAuthenticationToken.java
+++ b/cas/src/main/java/org/springframework/security/cas/authentication/CasAuthenticationToken.java
@@ -108,7 +108,7 @@ private CasAuthenticationToken(final Integer keyHash, final Object principal, fi
 	protected CasAuthenticationToken(Builder<?> builder) {
 		super(builder);
 		Assert.isTrue(!"".equals(builder.principal), "principal cannot be null or empty");
-		Assert.notNull(!"".equals(builder.credentials), "credentials cannot be null or empty");
+		Assert.isTrue(!"".equals(builder.credentials), "credentials cannot be null or empty");
 		Assert.notNull(builder.userDetails, "userDetails cannot be null");
 		Assert.notNull(builder.assertion, "assertion cannot be null");
 		this.keyHash = builder.keyHash;
diff --git a/cas/src/test/java/org/springframework/security/cas/authentication/CasAuthenticationTokenTests.java b/cas/src/test/java/org/springframework/security/cas/authentication/CasAuthenticationTokenTests.java
index 506497124d4..ee5b29962aa 100644
--- a/cas/src/test/java/org/springframework/security/cas/authentication/CasAuthenticationTokenTests.java
+++ b/cas/src/test/java/org/springframework/security/cas/authentication/CasAuthenticationTokenTests.java
@@ -34,6 +34,7 @@
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
+import static org.assertj.core.api.Assertions.assertThatNoException;
 
 /**
  * Tests {@link CasAuthenticationToken}.
@@ -182,4 +183,38 @@ public void toBuilderWhenApplyThenCopies() {
 		assertThat(authorities).containsExactlyInAnyOrder("FACTOR_ONE", "FACTOR_TWO");
 	}
 
+	@Test
+	public void toBuilderWhenPrincipalIsEmpty() {
+		final Assertion assertion = new AssertionImpl("test");
+		CasAuthenticationToken token = new CasAuthenticationToken("key", makeUserDetails(), "Password", this.ROLES,
+				makeUserDetails(), assertion);
+		assertThatIllegalArgumentException().isThrownBy(() -> token.toBuilder().principal(null).build());
+		assertThatIllegalArgumentException().isThrownBy(() -> token.toBuilder().principal("").build());
+	}
+
+	@Test
+	public void toBuilderWhenPrincipalIsNotEmpty() {
+		final Assertion assertion = new AssertionImpl("test");
+		CasAuthenticationToken token = new CasAuthenticationToken("key", makeUserDetails(), "Password", this.ROLES,
+				makeUserDetails(), assertion);
+		assertThatNoException().isThrownBy(() -> token.toBuilder().principal("principal").build());
+	}
+
+	@Test
+	public void toBuilderWhenCredentialsIsEmpty() {
+		final Assertion assertion = new AssertionImpl("test");
+		CasAuthenticationToken token = new CasAuthenticationToken("key", makeUserDetails(), "Password", this.ROLES,
+				makeUserDetails(), assertion);
+		assertThatIllegalArgumentException().isThrownBy(() -> token.toBuilder().credentials(null).build());
+		assertThatIllegalArgumentException().isThrownBy(() -> token.toBuilder().credentials("").build());
+	}
+
+	@Test
+	public void toBuilderWhenCredentialsIsNotEmpty() {
+		final Assertion assertion = new AssertionImpl("test");
+		CasAuthenticationToken token = new CasAuthenticationToken("key", makeUserDetails(), "Password", this.ROLES,
+				makeUserDetails(), assertion);
+		assertThatNoException().isThrownBy(() -> token.toBuilder().credentials("credentials").build());
+	}
+
 }