diff --git a/camp/camp-brooklyn/src/test/java/org/apache/brooklyn/camp/brooklyn/ConfigParametersYamlTest.java b/camp/camp-brooklyn/src/test/java/org/apache/brooklyn/camp/brooklyn/ConfigParametersYamlTest.java
index 5f277eec97..5db4601154 100644
--- a/camp/camp-brooklyn/src/test/java/org/apache/brooklyn/camp/brooklyn/ConfigParametersYamlTest.java
+++ b/camp/camp-brooklyn/src/test/java/org/apache/brooklyn/camp/brooklyn/ConfigParametersYamlTest.java
@@ -1181,6 +1181,46 @@ public void testConfigParameterConstraintObject() throws Exception {
         assertKeyEquals(newEntity, "testRequired", null, String.class, null, "myprefix-myVal");
     }
 
+    @Test
+    public void testConfigParameterConstraintOnOtherKeyWithAttributeWhenReady() throws Exception {
+        addCatalogItems(
+                "brooklyn.catalog:",
+                "  itemType: entity",
+                "  items:",
+                "  - id: entity-with-keys",
+                "    item:",
+                "      type: "+TestEntity.class.getName(),
+                "      brooklyn.parameters:",
+                "      - name: key1",
+                "        type: String",
+                "        constraints:",
+                "        - requiredUnless: key2",
+                "      - name: key2",
+                "        type: String",
+                "        constraints:",
+                "        - requiredUnless: key1");
+        
+        String yaml = Joiner.on("\n").join(
+                "services:",
+                "- type: entity-with-keys",
+                "  brooklyn.config:",
+                "    key1: $brooklyn:root().attributeWhenReady(\"myattribute\")");
+
+        Entity app = createStartWaitAndLogApplication(yaml);
+        TestEntity entity = (TestEntity) Iterables.getOnlyElement(app.getChildren());
+        
+        Predicate<?> constraint = entity.getEntityType().getConfigKey("key1").getConstraint();
+        assertEquals(constraint.toString(), "requiredUnless(\"key2\")");
+
+        // Rebind, and then check again that the config key is listed
+        Entity newApp = rebind();
+        TestEntity newEntity = (TestEntity) Iterables.getOnlyElement(newApp.getChildren());
+        
+        Predicate<?> newConstraint = newEntity.getEntityType().getConfigKey("key1").getConstraint();
+        assertEquals(newConstraint.toString(), "requiredUnless(\"key2\")");
+    }
+
+
     public static class PredicateRegexPojo implements Predicate<Object> {
         private String regex;
 
diff --git a/core/src/main/java/org/apache/brooklyn/core/config/ConfigConstraints.java b/core/src/main/java/org/apache/brooklyn/core/config/ConfigConstraints.java
index b0ed3f8271..bcc9a25e3a 100644
--- a/core/src/main/java/org/apache/brooklyn/core/config/ConfigConstraints.java
+++ b/core/src/main/java/org/apache/brooklyn/core/config/ConfigConstraints.java
@@ -300,7 +300,17 @@ public boolean apply(Object input, BrooklynObject context) {
             // would be nice to offer an explanation, but that will need a richer API or a thread local
             List<Object> vals = new ArrayList<>();
             for (String otherKeyName : otherKeyNames) {
-                vals.add(context.config().get(ConfigKeys.newConfigKey(Object.class, otherKeyName)));
+                // Use getNonBlocking, in case the value is an `attributeWhenReady` and the 
+                // attribute is not yet available - don't want to hang.
+                ConfigKey<Object> otherKey = ConfigKeys.newConfigKey(Object.class, otherKeyName);
+                BrooklynObjectInternal.ConfigurationSupportInternal configInternal = ((BrooklynObjectInternal) context).config();
+                
+                Maybe<?> maybeValue = configInternal.getNonBlocking(otherKey);
+                if (maybeValue.isPresent()) {
+                    vals.add(maybeValue.get());
+                } else {
+                    return true; // abort; assume valid
+                }
             }
             return test(input, vals);
         }