diff --git a/java/google/registry/model/domain/fee/BaseFee.java b/java/google/registry/model/domain/fee/BaseFee.java
index 17a0e411b..4e74c3948 100644
--- a/java/google/registry/model/domain/fee/BaseFee.java
+++ b/java/google/registry/model/domain/fee/BaseFee.java
@@ -28,7 +28,7 @@ import org.joda.time.Period;
 
 /** Base class for the fee and credit types. */
 @XmlTransient
-public class BaseFee extends ImmutableObject {
+public abstract class BaseFee extends ImmutableObject {
 
   /** Enum for when a fee is applied. */
   public enum AppliedType {
@@ -71,6 +71,13 @@ public class BaseFee extends ImmutableObject {
     return firstNonNull(refundable, true);
   }
 
+  /**
+   * According to the fee extension specification, a fee must always be non-negative, while a credit
+   * must always be negative. Essentially, they are the same thing, just with different sign.
+   * However, we need them to be separate classes for proper JAXB handling.
+   * 
+   * @see "https://tools.ietf.org/html/draft-brown-epp-fees-03#section-2.4"
+   */
   public BigDecimal getCost() {
     return cost;
   }
diff --git a/java/google/registry/model/domain/fee/Credit.java b/java/google/registry/model/domain/fee/Credit.java
index 1e34d3c36..ce64e0aac 100644
--- a/java/google/registry/model/domain/fee/Credit.java
+++ b/java/google/registry/model/domain/fee/Credit.java
@@ -14,6 +14,7 @@
 
 package google.registry.model.domain.fee;
 
+import static com.google.common.base.Preconditions.checkArgument;
 import static com.google.common.base.Preconditions.checkNotNull;
 
 import java.math.BigDecimal;
@@ -23,6 +24,7 @@ public class Credit extends BaseFee {
   public static Credit create(BigDecimal cost, String description) {
     Credit instance = new Credit();
     instance.cost = checkNotNull(cost);
+    checkArgument(instance.cost.signum() < 0);
     instance.description = description;
     return instance;
   }
diff --git a/java/google/registry/model/domain/fee/Fee.java b/java/google/registry/model/domain/fee/Fee.java
index d90cf2748..a64fcad3f 100644
--- a/java/google/registry/model/domain/fee/Fee.java
+++ b/java/google/registry/model/domain/fee/Fee.java
@@ -14,6 +14,7 @@
 
 package google.registry.model.domain.fee;
 
+import static com.google.common.base.Preconditions.checkArgument;
 import static com.google.common.base.Preconditions.checkNotNull;
 
 import com.google.common.collect.ImmutableList;
@@ -41,6 +42,7 @@ public class Fee extends BaseFee {
   public static Fee create(BigDecimal cost, String description) {
     Fee instance = new Fee();
     instance.cost = checkNotNull(cost);
+    checkArgument(instance.cost.signum() >= 0);
     instance.description = description;
     return instance;
   }