diff --git a/sfdx-source/apex-mocks/main/classes/fflib_ApexMocksUtils.cls b/sfdx-source/apex-mocks/main/classes/fflib_ApexMocksUtils.cls
index 3709b96c..5976e4e8 100644
--- a/sfdx-source/apex-mocks/main/classes/fflib_ApexMocksUtils.cls
+++ b/sfdx-source/apex-mocks/main/classes/fflib_ApexMocksUtils.cls
@@ -110,13 +110,12 @@ public class fflib_ApexMocksUtils
 	 * values on read-only fields by their name
 	 */
 	public static Object setReadOnlyFields(SObject objInstance, Type deserializeType, Map<String, Object> properties) {
+		JSONParser parser = JSON.createParser((JSON.serialize(objInstance)));
+		JSONGenerator combinedOutput = JSON.createGenerator(false);
 
-		Map<String, Object> mergedMap = new Map<String, Object>(objInstance.getPopulatedFieldsAsMap());
-		// Merge the values from the properties map into the fields already set on the object
-		mergedMap.putAll(properties);
-		// Serialize the merged map, and then deserialize it as the desired object type.
-		String jsonString = JSON.serializePretty(mergedMap);
-		return (SObject) JSON.deserialize(jsonString, deserializeType);
+		// parse the source object and inject new fields into the resulting JSON
+		streamTokens(parser, combinedOutput, new InjectFieldsEventHandler(properties) );
+		return JSON.deserialize(combinedOutput.getAsString(), deserializeType);
 	}
 
 	/**
@@ -183,6 +182,29 @@ public class fflib_ApexMocksUtils
 		}
 	}
 
+	/**
+	 * Monitors stream events for end of object for the current SObject
+	 *   then injects new fields into the stream
+	 */
+	private class InjectFieldsEventHandler implements JSONParserEvents
+	{
+		Map<String, Object> properties;
+
+		public InjectFieldsEventHandler(Map<String, Object> properties) {
+			this.properties = properties;
+		}
+
+		public void nextToken(JSONParser fromStream, Integer depth, JSONGenerator toStream) {
+			// Inject children?
+			JSONToken currentToken = fromStream.getCurrentToken();
+			if(depth == 1 && currentToken == JSONToken.END_OBJECT ) {
+				for(String field : properties.keySet()) {
+					toStream.writeObjectField(field, properties.get(field));
+				}
+			}
+		}
+	}
+
 	/**
 	 * Utility function to stream tokens from a reader to a write, while providing a basic eventing model
 	 */
diff --git a/sfdx-source/apex-mocks/test/classes/fflib_ApexMocksUtilsTest.cls b/sfdx-source/apex-mocks/test/classes/fflib_ApexMocksUtilsTest.cls
index cf613070..b21610d0 100644
--- a/sfdx-source/apex-mocks/test/classes/fflib_ApexMocksUtilsTest.cls
+++ b/sfdx-source/apex-mocks/test/classes/fflib_ApexMocksUtilsTest.cls
@@ -337,4 +337,47 @@ public class fflib_ApexMocksUtilsTest
 
 		System.Assert.areEqual(acc.Name, t.What.Name);
 	}
+
+	@isTest
+	static void setReadOnlyFields_WithChildren_FieldSetSuccessfully() {
+		// Given
+		Contact cont = new Contact(
+			Id = fflib_IDGenerator.generate(Contact.SObjectType),
+			LastName = 'ContactName'
+		);
+
+		Opportunity opp = new Opportunity(
+			Id = fflib_IDGenerator.generate(Opportunity.SObjectType),
+			Name = 'OpportunityName'
+		);
+
+		Account acc = new Account();
+
+		Id userId = fflib_IDGenerator.generate((new User()).getSObjectType());
+
+		acc = ((List<Account>) fflib_ApexMocksUtils.makeRelationship(
+			List<Account>.class,
+			new List<Account>{ acc },
+			Contact.AccountId,
+			new List<List<Contact>>{ new List<Contact>{cont} }
+		))[0];
+
+		acc = ((List<Account>) fflib_ApexMocksUtils.makeRelationship(
+			List<Account>.class,
+			new List<Account>{ acc },
+			Opportunity.AccountId,
+			new List<List<Opportunity>>{ new List<Opportunity>{opp} }
+		))[0];
+
+		// When
+		Test.startTest();
+		acc = (Account)fflib_ApexMocksUtils.setReadOnlyFields(
+				acc,
+				Account.class,
+				new Map<SObjectField, Object>{Account.CreatedById => userId}
+		);
+		Test.stopTest();
+
+		Assert.areEqual(userId, acc.CreatedById);
+	}
 }