diff --git a/Core/src/main/java/com/songoda/core/configuration/ConfigEntry.java b/Core/src/main/java/com/songoda/core/configuration/ConfigEntry.java
index 52cc914f..9e4e8fb6 100644
--- a/Core/src/main/java/com/songoda/core/configuration/ConfigEntry.java
+++ b/Core/src/main/java/com/songoda/core/configuration/ConfigEntry.java
@@ -67,7 +67,7 @@ public interface ConfigEntry {
      * @param valueConverter    A function that converts the old version's value to a new one, or null if it didn't change
      */
     @Contract("_, null, null -> fail; _, _, _ -> this")
-    ConfigEntry withUpgradeStep(int version, @Nullable String keyInGivenVersion, @Nullable Function<Object, Object> valueConverter);
+    ConfigEntry withUpgradeStep(int version, @Nullable String keyInGivenVersion, @Nullable Function<@Nullable Object, @Nullable Object> valueConverter);
 
     default @Nullable String getString() {
         return getStringOr(null);
diff --git a/Core/src/main/java/com/songoda/core/configuration/songoda/SongodaYamlConfig.java b/Core/src/main/java/com/songoda/core/configuration/songoda/SongodaYamlConfig.java
index 78287078..40cca8bb 100644
--- a/Core/src/main/java/com/songoda/core/configuration/songoda/SongodaYamlConfig.java
+++ b/Core/src/main/java/com/songoda/core/configuration/songoda/SongodaYamlConfig.java
@@ -79,8 +79,8 @@ public class SongodaYamlConfig extends YamlConfiguration {
             this.save();
 
             return true;
-        } catch (IOException e) {
-            this.logger.log(Level.SEVERE, "Failed to load config file: " + this.file.getPath(), e);
+        } catch (IOException ex) {
+            this.logger.log(Level.SEVERE, "Failed to load config file: " + this.file.getPath(), ex);
         }
 
         return false;
@@ -134,8 +134,8 @@ public class SongodaYamlConfig extends YamlConfiguration {
         try (Reader reader = new FileReader(this.file)) {
             load(reader);
         } catch (FileNotFoundException ignore) {
-        } catch (IOException e) {
-            throw new IOException("Unable to load '" + this.file.getPath() + "'", e);
+        } catch (IOException ex) {
+            throw new IOException("Unable to load '" + this.file.getPath() + "'", ex);
         }
     }
 
@@ -144,8 +144,8 @@ public class SongodaYamlConfig extends YamlConfiguration {
 
         try (Writer writer = new FileWriter(this.file)) {
             super.save(writer);
-        } catch (IOException e) {
-            throw new IOException("Unable to save '" + this.file.getPath() + "'", e);
+        } catch (IOException ex) {
+            throw new IOException("Unable to save '" + this.file.getPath() + "'", ex);
         }
     }
 
diff --git a/Core/src/main/java/com/songoda/core/configuration/yaml/YamlConfigEntry.java b/Core/src/main/java/com/songoda/core/configuration/yaml/YamlConfigEntry.java
index dc075188..d29da1a9 100644
--- a/Core/src/main/java/com/songoda/core/configuration/yaml/YamlConfigEntry.java
+++ b/Core/src/main/java/com/songoda/core/configuration/yaml/YamlConfigEntry.java
@@ -51,7 +51,7 @@ public class YamlConfigEntry implements WriteableConfigEntry {
     }
 
     @Override
-    public ConfigEntry withUpgradeStep(int version, @Nullable String keyInGivenVersion, @Nullable Function<Object, Object> valueConverter) {
+    public ConfigEntry withUpgradeStep(int version, @Nullable String keyInGivenVersion, @Nullable Function<@Nullable Object, @Nullable Object> valueConverter) {
         if (keyInGivenVersion == null && valueConverter == null) {
             throw new IllegalArgumentException("You must provide either a key or a value converter");
         }