diff --git a/src/main/java/net/openhft/chronicle/map/ChronicleMapBuilder.java b/src/main/java/net/openhft/chronicle/map/ChronicleMapBuilder.java
index 4d1b8f0b6..cfa79e85c 100755
--- a/src/main/java/net/openhft/chronicle/map/ChronicleMapBuilder.java
+++ b/src/main/java/net/openhft/chronicle/map/ChronicleMapBuilder.java
@@ -296,8 +296,7 @@ public final class ChronicleMapBuilder<K, V> implements
      */
     @Override
     public ChronicleMapBuilder<K, V> averageKeySize(double averageKeySize) {
-        if (keyBuilder.sizeIsStaticallyKnown)
-            return this;
+        checkSizeIsStaticallyKnown(keyBuilder, "Key");
         checkAverageSize(averageKeySize, "key");
         this.averageKeySize = averageKeySize;
         averageKey = null;
@@ -319,8 +318,7 @@ public final class ChronicleMapBuilder<K, V> implements
     @Override
     public ChronicleMapBuilder<K, V> averageKey(K averageKey) {
         Objects.requireNonNull(averageKey);
-        if (keyBuilder.sizeIsStaticallyKnown)
-            return this;
+        checkSizeIsStaticallyKnown(keyBuilder, "Key");
         this.averageKey = averageKey;
         sampleKey = null;
         averageKeySize = UNDEFINED_DOUBLE_CONFIG;
@@ -384,8 +382,7 @@ public final class ChronicleMapBuilder<K, V> implements
      * @see #actualChunkSize(int)
      */
     public ChronicleMapBuilder<K, V> averageValueSize(double averageValueSize) {
-        if (valueBuilder.sizeIsStaticallyKnown)
-            return this;
+        checkSizeIsStaticallyKnown(valueBuilder, "Value");
         checkAverageSize(averageValueSize, "value");
         this.averageValueSize = averageValueSize;
         averageValue = null;
@@ -427,14 +424,21 @@ public final class ChronicleMapBuilder<K, V> implements
      */
     public ChronicleMapBuilder<K, V> averageValue(V averageValue) {
         Objects.requireNonNull(averageValue);
-        if (valueBuilder.sizeIsStaticallyKnown)
-            return this;
+        checkSizeIsStaticallyKnown(valueBuilder, "Value");
         this.averageValue = averageValue;
         sampleValue = null;
         averageValueSize = UNDEFINED_DOUBLE_CONFIG;
         return this;
     }
 
+    private static void checkSizeIsStaticallyKnown(SerializationBuilder builder, String role) {
+        if (builder.sizeIsStaticallyKnown) {
+            throw new IllegalStateException("Size of " + builder.tClass +
+                    " instances is constant and statically known, shouldn't be specified via " +
+                    "average" + role + "Size() or average" + role + "() methods");
+        }
+    }
+
     private static void checkAverageSize(double averageSize, String role) {
         if (averageSize <= 0 || isNaN(averageSize) ||
                 Double.isInfinite(averageSize)) {
diff --git a/src/test/java/net/openhft/chronicle/map/HColl423Test.java b/src/test/java/net/openhft/chronicle/map/HColl423Test.java
new file mode 100644
index 000000000..b68afced4
--- /dev/null
+++ b/src/test/java/net/openhft/chronicle/map/HColl423Test.java
@@ -0,0 +1,38 @@
+/*
+ *      Copyright (C) 2015  higherfrequencytrading.com
+ *
+ *      This program is free software: you can redistribute it and/or modify
+ *      it under the terms of the GNU Lesser General Public License as published by
+ *      the Free Software Foundation, either version 3 of the License.
+ *
+ *      This program is distributed in the hope that it will be useful,
+ *      but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *      MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *      GNU Lesser General Public License for more details.
+ *
+ *      You should have received a copy of the GNU Lesser General Public License
+ *      along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package net.openhft.chronicle.map;
+
+import org.junit.Test;
+
+public class HColl423Test {
+
+    @Test
+    public void hColl423Test() {
+        try {
+            ChronicleMap<String, Integer> map = ChronicleMap
+                    .of(String.class, Integer.class)
+                    .averageKeySize(128)
+                    .averageValueSize(100)
+                    .entries(2_000_000L)
+                    .maxBloatFactor(1_000.0)
+                    .create();
+            throw new AssertionError("should throw IllegalStateException");
+        } catch (IllegalStateException expected) {
+            // do nothing
+        }
+    }
+}