diff --git a/formula/src/test/java/com/instacart/formula/FormulaRuntimeTest.kt b/formula/src/test/java/com/instacart/formula/FormulaRuntimeTest.kt
index 2920f3ce..f3491342 100644
--- a/formula/src/test/java/com/instacart/formula/FormulaRuntimeTest.kt
+++ b/formula/src/test/java/com/instacart/formula/FormulaRuntimeTest.kt
@@ -1101,33 +1101,37 @@ class FormulaRuntimeTest(val runtime: TestableRuntime, val name: String) {
     }
 
     @Test fun `adding duplicate child throws an exception`() {
+        val testKey = TestKey("1")
         val result = Try {
             val formula = DynamicParentFormula()
             runtime.test(formula, Unit)
-                .output { addChild(TestKey("1")) }
-                .output { addChild(TestKey("1")) }
+                .output { addChild(testKey) }
+                .output { addChild(testKey) }
         }
 
         val error = result.errorOrNull()?.cause
         assertThat(error).isInstanceOf(IllegalStateException::class.java)
+        assertThat(error?.message).isEqualTo("There already is a child with same key: $testKey. Override [Formula.key] function.")
     }
 
     @Test fun `adding duplicate child logs an exception`() {
         val logs = mutableListOf<String>()
-        RxJavaRuntime.setDefaultErrorHandler { logs.add(it.localizedMessage) }
+        RxJavaRuntime.setDefaultErrorHandler { logs.add(it.message.orEmpty()) }
+        val testKey = TestKey("1")
 
         val result = Try {
             val formula = DynamicParentFormula()
             runtime.test(formula, Unit)
-                .output { addChild(TestKey("1")) }
-                .output { addChild(TestKey("1")) }
+                .output { addChild(testKey) }
+                .output { addChild(testKey) }
         }
 
         val error = result.errorOrNull()?.cause
         assertThat(error).isNull()
         assertThat(logs).hasSize(1)
         val log = logs.first()
-        assertThat(log).isEqualTo("Duplicate child key: TestKey(id=1)")
+        assertThat(log).isEqualTo("There already is a child with same key: $testKey. Override [Formula.key] function.")
+        RxJavaRuntime.setDefaultErrorHandler(null)
     }
 
     @Test