diff --git a/cas-client-core/src/main/java/org/jasig/cas/client/util/AbstractCasFilter.java b/cas-client-core/src/main/java/org/jasig/cas/client/util/AbstractCasFilter.java
index 9b2da82..3b8ea19 100644
--- a/cas-client-core/src/main/java/org/jasig/cas/client/util/AbstractCasFilter.java
+++ b/cas-client-core/src/main/java/org/jasig/cas/client/util/AbstractCasFilter.java
@@ -101,6 +101,7 @@ public abstract class AbstractCasFilter extends AbstractConfigurationFilter {
         CommonUtils.assertNotNull(this.artifactParameterName, "artifactParameterName cannot be null.");
         CommonUtils.assertNotNull(this.serviceParameterName, "serviceParameterName cannot be null.");
         CommonUtils.assertTrue(CommonUtils.isNotEmpty(this.serverName) || CommonUtils.isNotEmpty(this.service), "serverName or service must be set.");
+        CommonUtils.assertTrue(CommonUtils.isBlank(this.serverName) || CommonUtils.isBlank(this.service), "serverName and service cannot both be set.  You MUST ONLY set one.");
     }
 
     // empty implementation as most filters won't need this.