diff --git a/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/MappingMongoConverter.java b/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/MappingMongoConverter.java
index 8dfa262f5..876e289a7 100644
--- a/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/MappingMongoConverter.java
+++ b/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/MappingMongoConverter.java
@@ -591,7 +591,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 
 			if (conversions.isSimpleType(key.getClass())) {
 
-				String simpleKey = potentiallyEscapeMapKey(key.toString());
+				String simpleKey = prepareMapKey(key.toString());
 				dbObject.put(simpleKey, value != null ? createDBRef(value, property) : null);
 
 			} else {
@@ -643,11 +643,13 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 	protected DBObject writeMapInternal(Map<Object, Object> obj, DBObject dbo, TypeInformation<?> propertyType) {
 
 		for (Map.Entry<Object, Object> entry : obj.entrySet()) {
+
 			Object key = entry.getKey();
 			Object val = entry.getValue();
+
 			if (conversions.isSimpleType(key.getClass())) {
 
-				String simpleKey = potentiallyEscapeMapKey(potentiallyConvertMapKey(key));
+				String simpleKey = prepareMapKey(key);
 				if (val == null || conversions.isSimpleType(val.getClass())) {
 					writeSimpleInternal(val, dbo, simpleKey);
 				} else if (val instanceof Collection || val.getClass().isArray()) {
@@ -668,17 +670,19 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		return dbo;
 	}
 
-	private String potentiallyConvertMapKey(Object key) {
+	/**
+	 * Prepares the given {@link Map} key to be converted into a {@link String}. Will invoke potentially registered custom
+	 * conversions and escape dots from the result as they're not supported as {@link Map} key in MongoDB.
+	 * 
+	 * @param key must not be {@literal null}.
+	 * @return
+	 */
+	private String prepareMapKey(Object key) {
 
-		if (key instanceof String) {
-			return (String) key;
-		}
+		Assert.notNull(key, "Map key must not be null!");
 
-		if (conversions.hasCustomWriteTarget(key.getClass(), String.class)) {
-			return (String) getPotentiallyConvertedSimpleWrite(key);
-		}
-
-		return key.toString();
+		String convertedKey = potentiallyConvertMapKey(key);
+		return potentiallyEscapeMapKey(convertedKey);
 	}
 
 	/**
@@ -703,6 +707,22 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		return source.replaceAll("\\.", mapKeyDotReplacement);
 	}
 
+	/**
+	 * Returns a {@link String} representation of the given {@link Map} key
+	 * 
+	 * @param key
+	 * @return
+	 */
+	private String potentiallyConvertMapKey(Object key) {
+
+		if (key instanceof String) {
+			return (String) key;
+		}
+
+		return conversions.hasCustomWriteTarget(key.getClass(), String.class) ? (String) getPotentiallyConvertedSimpleWrite(key)
+				: key.toString();
+	}
+
 	/**
 	 * Translates the map key replacements in the given key just read with a dot in case a map key replacement has been
 	 * configured.
diff --git a/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/MappingMongoConverterUnitTests.java b/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/MappingMongoConverterUnitTests.java
index b58bab1af..7ca8ec83f 100644
--- a/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/MappingMongoConverterUnitTests.java
+++ b/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/MappingMongoConverterUnitTests.java
@@ -77,6 +77,7 @@ import org.springframework.data.mapping.model.MappingInstantiationException;
 import org.springframework.data.mongodb.core.DBObjectTestUtils;
 import org.springframework.data.mongodb.core.convert.DBObjectAccessorUnitTests.NestedType;
 import org.springframework.data.mongodb.core.convert.DBObjectAccessorUnitTests.ProjectingType;
+import org.springframework.data.mongodb.core.convert.MappingMongoConverterUnitTests.ClassWithMapUsingEnumAsKey.FooBarEnum;
 import org.springframework.data.mongodb.core.geo.Sphere;
 import org.springframework.data.mongodb.core.mapping.Document;
 import org.springframework.data.mongodb.core.mapping.Field;
@@ -1967,7 +1968,6 @@ public class MappingMongoConverterUnitTests {
 		assertThat(converter.read(TypeWithLocalDateTime.class, result).date, is(reference));
 	}
 
-
 	/**
 	 * @see DATAMONGO-1128
 	 */
@@ -2009,6 +2009,7 @@ public class MappingMongoConverterUnitTests {
 	 * @see DATAMONGO-1118
 	 */
 	@Test
+	@SuppressWarnings("unchecked")
 	public void convertsMapKeyUsingCustomConverterForAndBackwards() {
 
 		MappingMongoConverter converter = new MappingMongoConverter(resolver, mappingContext);
@@ -2017,13 +2018,13 @@ public class MappingMongoConverterUnitTests {
 		converter.afterPropertiesSet();
 
 		ClassWithMapUsingEnumAsKey source = new ClassWithMapUsingEnumAsKey();
-		source.map = new HashMap<MappingMongoConverterUnitTests.FooBarEnum, String>();
+		source.map = new HashMap<FooBarEnum, String>();
 		source.map.put(FooBarEnum.FOO, "wohoo");
 
 		DBObject target = new BasicDBObject();
 		converter.write(source, target);
 
-		assertThat(converter.read(ClassWithMapUsingEnumAsKey.class, target).map, equalTo(source.map));
+		assertThat(converter.read(ClassWithMapUsingEnumAsKey.class, target).map, is(source.map));
 	}
 
 	/**
@@ -2037,7 +2038,7 @@ public class MappingMongoConverterUnitTests {
 		converter.afterPropertiesSet();
 
 		ClassWithMapUsingEnumAsKey source = new ClassWithMapUsingEnumAsKey();
-		source.map = new HashMap<MappingMongoConverterUnitTests.FooBarEnum, String>();
+		source.map = new HashMap<FooBarEnum, String>();
 		source.map.put(FooBarEnum.FOO, "spring");
 		source.map.put(FooBarEnum.BAR, "data");
 
@@ -2370,11 +2371,12 @@ public class MappingMongoConverterUnitTests {
 		Optional<LocalDateTime> localDateTime = Optional.empty();
 	}
 
-	static enum FooBarEnum {
-		FOO, BAR;
-	}
-
 	static class ClassWithMapUsingEnumAsKey {
+
+		static enum FooBarEnum {
+			FOO, BAR;
+		}
+
 		Map<FooBarEnum, String> map;
 	}
 
@@ -2383,13 +2385,13 @@ public class MappingMongoConverterUnitTests {
 
 		@Override
 		public String convert(FooBarEnum source) {
+
 			if (source == null) {
 				return null;
 			}
 
 			return FooBarEnum.FOO.equals(source) ? "foo-enum-value" : "bar-enum-value";
 		}
-
 	}
 
 	@ReadingConverter
@@ -2411,8 +2413,5 @@ public class MappingMongoConverterUnitTests {
 
 			throw new ConversionNotSupportedException(source, String.class, null);
 		}
-
 	}
-
-
 }